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LIBRARY OF CONGRESS 


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declassified 

By authority Secretary of 

SEP 7 1960 

Defense memo 2 August 1960 
SUMMARY TECHNICAL REPORT LIBRARY OF CONGRESS 
OE THE 

NATIONAL DEEENSE RESEARCH COMMITTEE 


0 




This document contains information affecting the national defense of 
the United States within the meaning of the Espionage Act, 50 U. S. C., 
31 and 32, as amended. Its transmission or the revelation of its contents 
in any manner to an unauthorized person is prohibited by law. 

This volume is classified in accordance with security 
regulations of the War and Navy Departments because certain chapters 
contain material which was at the date of printing. 

Other chapters may have had a lower classification or none. The reader 
is advised to consult the War and Navy agencies listed on the reverse 
of this page for the current classification of any material. 



Manuscript and illustrations for this volume were prepared 
for publication by the Summary Reports Group of the 
Columbia University Division of War Research under con- 
tract OEMsr-1131 with the Office of Scientific Research and 
Development. This volume was printed and bound by the 
Columbia University Press. 

Distribution of the Summary Technical Report of NDRC has 
been made by the War and Navy Departments. Inquiries con- 
cerning the availability and distribution of the Summary 
Technical Report volumes and microfilmed and other refer- 
ence material should be addressed to the War Department 
Library, Room lA-522, The Pentagon, Washington 25, D. C., 
or to the Office of Naval Research, Navy Department, Atten- 
tion: Reports and Documents Section, Washington 25, D. C. 

Copy No. 

238 


This volume, like the seventy others of the Summary Techni- 
cal Report of NDRC, has been written, edited, and printed 
under great pressure. Inevitably there are errors which have 
slipped past Division readers and proofreaders. There may 
be errors of fact not known at time of printing. The author 
has not been able to follow through his writing to the final 
page proof. 

Please report errors to : 

JOINT RESEARCH AND DEVELOPMENT BOARD 
PROGRAMS DIVISION (STR ERRATA) 

WASHINGTON 25, D. C. 

A master errata sheet will be compiled from these reports 
and sent to recipients of the volume. Your help will make 
this book more useful to other readers and will be of great 
value in preparing any revisions. 



SUMMARY TECHNICAL REPORT OF DIVISION 6, NDRC 


VOLUME 9 


DECLASSIFIED 
By authority Secretary of 


SEP 7 1960 


RECOGNITION O'P se memo 2 August 1960 

LIBRARY OF CONGRESS 

UNDERWATER SOUNDS 


LC REGULATION: BEFORE SERVICING 
OR REPRODUCING ANY PART OF THIS 
DOCUMENT, ALL CLASSIFICATION 
MARKINGS MUST BE CANCELLED. 

OFFICE OF SCIENTIFIC RESEARCH AND DEVELOPMENT 
VANNEVAR BUSH, DIRECTOR 

NATIONAL DEFENSE RESEARCH COMMITTEE 
JAMES B. CONANT, CHAIRMAN 

DIVISION 6 
JOHN T. TATE, CHIEF 


WASHINGTON, D. C., 1946 



NATIONAL DEFENSE RESEARCH COMMITTEE 


James B. Conant, Chairman 
Richard C. Tolman, Vice Chairman 
Roger Adams Army Representative ^ 

Frank B. Jewett Navy Representative - 

Karl T. Compton Commissioner of Patents ^ 

•!. •? Executive Secretary 

. * ' . > . • 

Army re2Jresentatives in order of service: 

Maj. Gen. G. V. Strong Col. L. A. Denson 

Maj. Gen. R. C. Moore Col. P. R. Faymonville 

Maj. Gen. C. C. Williams Brig. Gen. E. A. Regnier 

Brig. Gen. W. A. Wood, Jr. Col. M. M. Irvine 

Col. E. A. Routheau 


-Navy rejyresentatives in order of service: 

Rear Adm. H. G. Bowen Rear Adm. J. A. Purer 
Capt. Lybrand P. Smith Rear Adm. A. H. Van Keuren 
Commodore H. A. Schade 
^Commissioners of Patents in order of service : 
Conway P. Coe Casper W. Ooms 


NOTES ON THE ORGANIZATION OF NDRC 


The duties of the National Defense Research Committee 
were (1) to recommend to the Director of OSRD suitable 
projects and research programs on the instrumentalities 
of warfare, together with contract facilities for carrying 
out these projects and programs, and (2) to administer 
the technical and scientific work of the contracts. More 
specifically, NDRC functioned by initiating research 
projects on requests from the Army or the Navy, or on 
requests from an allied government transmitted through 
the Liaison Office of OSRD, or on its own considered ini- 
tiative as a result of the experience of its members. Pro- 
posals prepared by the Division, Panel, or Committee for 
research contracts for performance of the work involved 
in such projects were first reviewed by NDRC, and if 
approved, recommended to the Director of OSRD. Upon 
approval of a proposal by the Director, a contract per- 
mitting maximum flexibility of scientific effort was ar- 
ranged. The business aspects of the contract, including 
such matters as materials, clearances, vouchers, patents, 
priorities, legal matters, and administration of patent 
matters were handled by the Executive Secretary of 
OSRD. 

Originally NDRC administered its work through five 
divisions, each headed by one of the NDRC members. 
These were: 

Division A — Armor and Ordnance 
Division B — Bombs, Fuels, Gases, & Chemical Prob- 
lems 

Division C — Communication and Transportation 
Division D — Detection, Controls, and Instruments 
Division E — Patents and Inventions 


In a reorganization in the fall of 1942, twenty-three 
administrative divisions, panels, or committees were 
created, each with a chief selected on the basis of his 
outstanding work in the particular field. The NDRC 
members then became a reviewing and advisory group 
to the Director of OSRD. The final organization was as 
follows: 

Division 1 — Ballistic Research 

Division 2 — Effects of Impact and Explosion 

Division 3 — Rocket Ordnance 

Division 4 — Ordnance Accessories 

Division 5 — New Missiles 

Division 6 — Sub-Surface Warfare 

Division 7 — Fire Control 

Division 8 — Explosives 

Division 9 — Chemistry 

Division 10 — Absorbents and Aerosols 

Division 11 — Chemical Engineering 

Division 12 — Transportation 

Division 13 — Electrical Communication 

Division 14 — Radar 

Division 15 — Radio Coordination 

Division 16 — Optics and Camouflage 

Division 17 — Physics 

Division 18 — War Metallurgy 

Division 19 — Miscellaneous 

Applied Mathematics Panel 

Applied Psychology Panel 

Committee on Propagation 

Tropical Deterioration Administrative Committee 


iv 



DECLASSIFIED 


NDRC FOREWORD 


By authority Secretary of 


A s EVENTS of the years preceding 1940 re- 
. vealed more and more clearly the serious- 
ness of the world situation, many scientists in 
this country came to realize the need of or- 
ganizing scientific research for service in a 
national emergency. Recommendations which 
they made to the White House were given 
careful and sympathetic attention, and as a 
result the National Defense Research Commit- 
tee [NDRC] was formed by Executive Order 
of the President in the summer of 1940. The 
members of NDRC, appointed by the Presi- 
dent, were instructed to supplement the work 
of the Army and the Navy in the development 
of the instrumentalities of war. A year later, 
upon the establishment of the Office of Sci- 
entific Research and Development [OSRD], 
NDRC became one of its units. 

The Summary Technical Report of NDRC is 
a conscientious effort on the part of NDRC to 
summarize and evaluate its work and to pre- 
sent it in a useful and permanent form. It 
comprises some seventy volumes broken into 
groups corresponding to the NDRC Divisions, 
Panels, and Committees. 

The Summary Technical Report of each Divi- 
sion, Panel, or Committee is an integral survey 
of the work of that group. The first volume of 
each group’s report contains a summary of the 
report, stating the problems presented and the 
philosophy of attacking them and summarizing 
the results of the research, development, and 
training activities undertaken. Some volumes 
may be “state of the art” treatises covering 
subjects to which various research groups have 
contributed information. Others may contain 
descriptions of devices developed in the labora- 
tories. A master index of all these divisional, 
panel, and committee reports which together 
constitute the Summary Technical Report of 
NDRC is contained in a separate volume, which 
also includes the index of a microfilm record of 
pertinent technical laboratory reports and ref- 
erence material. 

Some of the NDRC-sponsored researches 
which had been declassified by the end of 1945 
were of sufficient popular interest that it was 
found desirable to report them in the form of 
monographs, such as the series on radar by 
Division 14 and the monograph on sampling 
inspection by the Applied Mathematics Panel. 


Since the material treated in them is not dupli- 
cated in the Summary g’^^h(j[ical|0]g^ort of 
NDRC, the monographs are an important part 
of the story of these aspects 

In contrast to-^ffiS Infor mation on radar, 
which is of widesJ^^p^A^R'fe^^ sGfilNBfe^SSf 
which is released to the public, the research on 
subsurfa cLGvi i S£l^lLA1S6(^y ^BlSRiVIClNG 

of generaORitil®ilRI0DOfifcl^rQflLWKt®3A®1^0F THIS 
As a coniilQfiMNiflNiE, lAJilrtQifiAISSfFiRATrON 


found ali dHi<Haai»Qg MUSISMSiriaift^^lahBD: 
cal Report, which runs to over twenty volumes. 
The extent of the work of a Division cannot 
therefore be judged solely by the number of 
volumes devoted to it in the Summary Techni- 
cal Report of NDRC : account must be taken of 
the monographs and available reports published 
elsewhere. 

Any great cooperative endeavor must stand 
or fall with the will and integrity of the men 
engaged in it. This fact held true for NDRC 
from its inception, and for Division 6 under 
the leadership of Dr. John T. Tate. To Dr. Tate 
and the men who worked with him — some as 
members of Division 6, some as representatives 
of the Division’s contractors — belongs the sin- 
cere gratitude of the Nation for a difficult and 
often dangerous job well done. Their efforts 
contributed significantly to the outcome of our 
naval operations during the war and richly 
deserved the warm response they received from 
the Navy. In addition, their contributions to 
the knowledge of the ocean and to the art of 
oceanographic research will assuredly speed 
peacetime investigations in this field and bring 
rich benefits to all mankind. 

The Summary Technical Report of Division 
6, prepared under the direction of the Division 
Chief and authorized by him for publication, not 
only presents the methods and results of widely 
varied research and development programs but 
is essentially a record of the unstinted loyal 
cooperation of able men linked in a common 
effort to contribute to the defense of their Na- 
tion. To them all we extend our deep apprecia- 
tion. 


Vannevar Bush, Director 
Office of Scientific Research and Development 


J. B. CONANT, Chairman 
National Defense Research Committee 


^ESPRiCTED 


V 



FOREWORD 


M ethods not involving the human ear have 
been developed for the detection and rec- 
ognition of underwater sounds. These, how- 
ever, generally supplement rather than sup- 
plant aural recognition. In the past the human 
ear has been most generally used as a detector 
of underwater sounds, and there is every rea- 
son to believe that aural recognition of under- 
water sounds will continue to be of large im- 
portance in subsurface warfare. 

The work described in this report was a por- 
tion of the research program originally pro- 
posed in 1941, which came to include an in- 
vestigatio;r> of all the factors involved not only 
in the detection of submerged or partially sub- 
merged submarines but also in the detection of 
surface craft. The matter presented is lim- 
ited to a summary of present knowledge of 
aural recognition of underwater sounds, and 
is closely related to three other reports in the 
Division 6 Summary Report Series; namely. 
Volume 6A, Military Oceanography, by the 
Woods Hole Oceanographic Institution, Volume 
7, Principles of Undenvater Sound, by Dr. Carl 
Eckart, and Volume 8, Physics of Sound in the 
Sea, by the staff of the Sonar Analysis Group. 
The physical listening devices developed by the 
Division are not described in Volume 9 but may 
be found in Volume 14 of this series. Sonar 
Listening Systems. 


The experimental work of the Division was 
undertaken principally at its San Diego Labo- 
ratory under the contract with the University 
of California. Studies of experimental results 
and their application were continued by the 
Sonar Analysis Group operating until lately 
under a Division contract with Columbia Uni- 
versity. 

It was most fortunate that in undertaking 
an investigation of aural recognition and its 
application to subsurface warfare the Division 
had available the results of previous research 
on performance of the ear. In this connection 
special mention should be made of the work 
of Dr. Harvey Fletcher and his associates in 
the Bell Telephone Laboratories, Drs. S. S. 
Stevens and H. Davis at Harvard University, 
as well as others who continued to make sub- 
stantial contributions to the field of psycho- 
acoustics. 

The preparation of this report was under- 
taken by Dr. A. Spector, under the general 
supervision of Dr. Lyman Spitzer, Jr., Director 
of the Sonar Analysis Group of the Bureau of 
Ships. To them the Division expresses its deep 
appreciation. 

John T. Tate 
Chief, Division 6 


vii 





o 


PREFACE 


S ounds carry well in the sea, better than they 
do in air, and enormously better than do 
electromagnetic disturbances such as light. Un- 
derwater sounds are therefore well adapted to 
providing information about relatively distant 
objects and events on or beneath the surface of 
the sea. Thus, the detection of such sounds has 
important uses in subsurface warfare. 

The detection of sound requires, in addition 
to various types of receiving and amplifying 
equipment, a detector which can recognize when 
a particular sound is present. To date, the hu- 
man ear has been most generally used as a de- 
tector of underwater sounds, and its perform- 
ance has received the greatest amount of study. 
This volume summarizes present knowledge 
concerning the aural recognition of underwater 
sounds. Information on the factors affecting 
aural recognition, and ability to predict whether 
or not a particular sound will be recognized 
under various conditions are useful in a num- 
ber of ways, as, for example, in the training 
and selection of sonar operators, in the design 
and operation of gear, and in estimation of the 
maximum effective operating range of the sonar 
gear in each specific situation. 

Research in this field during the war has been 
primarily practical in nature. Emphasis has 
been placed on specific answers to questions of 
operational importance, rather than on a broad 
understanding of the basic factors involved. 
For the most effective use of these results, how- 
ever, and for planning future research in this 
field, a more basic approach is necessary. For 
this reason current theory on the performance 
of the ear, a subject known as psychoacoustics, 
has been introduced into the discussion wher- 
ever it seemed profitable. While this theory, 
developed by Fletcher, Stevens, and others, has 
not yet reached the point where it permits con- 
clusive predictions of the behavior of the ear, 
it does indicate a certain coherence among the 
observations that would otherwise not be ap- 
parent. Even where the theory seems incon- 
sistent with the observed results, the theoretical 
discussion may be profitable in suggesting a new 
experimental or theoretical approach. 


While the basic theory of the performance of 
the ear has been kept in mind in the prepara- 
tion of this volume, a deliberate attempt has 
been made to focus attention also on the many 
practical aspects of the information discussed 
here. Thus, in the discussion of the data, many 
points are brought out which are not directly 
relevant to results on the performance of the 
ear, but which bear on the application of these 
results to the most effective use of sonar in 
subsurface warfare. It is hoped that the re- 
sultant volume may be of interest both to the 
technical worker in psychoacoustics and to the 
engineer interested in the application of psycho- 
acoustic data to the design and operation of 
equipment. While the discussion is relatively 
exhaustive, the general level of the writing has 
been made sufficiently elementary so that it 
may be understood by those with little experi- 
ence either in mathematics or in sonar research. 

In reading this volume several important dis- 
tinctions should be kept in mind. The most 
important is that between “wanted” sounds, 
for which the operator is listening, and “un- 
wanted” sounds, or interference tending to 
mask the wanted sounds. A wanted sound is 
frequently called a signal, while the many un- 
wanted sounds are grouped together under the 
heading background. Whether a particular 
sound is classified as signal or background de- 
pends on the circumstances. For example, the 
sounds produced on board a submarine add to 
the background when the equipment mounted 
on the submarine is being used for listening to 
enemy vessels. When these local sounds are be- 
ing monitored by the submarine, however, they 
constitute the signal. Similarly, sounds from 
a submarine are the signal used by listening 
gear on an antisubmarine vessel, but form part 
of the background when echoes are used to 
measure the range to the submarine. 

A second, less fundamental distinction is that 
between local sounds, associated with the sonar 
gear and its mounting, and more distant sounds, 
produced by sources far away. Local sounds, 
including electrical noise in the hydrophone 
circuits, motion of water around the hydro- 



IX 


X 


PREFACE 


phone or ship (if the hydrophone is mounted 
on board ship), or noise produced by machin- 
ery on board ship, are frequently grouped to- 
gether under the heading self -noise, and gener- 
ally constitute much of the unwanted noise 
background. Sounds from distant objects in- 
clude many generally unwanted components, 
such as noise from whitecaps forming on the 
sea surface, or sound scattered from irregulari- 
ties on the bottom. In addition, some distant 
sounds, such as the sound generated or reflected 
by surface vessels, submarines, torpedoes and 
the like, usually are the signals which are to be 
recognized. These sounds may be used to pro- 
vide information about the bearing and nature 
of the sound source, and under certain condi- 
tions can be used to determine the speed, range, 
course, change of course, and other maneuvers 
of the source. 

The most important application of sonar 
hitherto has involved the use of shipboard- 
mounted gear to detect other ships, either on 
the surface or submerged. In the discussion of 
the observations, given in this volume, this 
situation is usually assumed. It should be em- 
phasized, however, that the basic results have 
a much wider validity. Sound gear can also 
be towed, suspended from a buoy, or installed 
on or under a harbor bottom. The received 
sounds may be presented to the ear not only on 
board ship but also in aircraft or at shore sta- 
tions. While the tactical problems are quite 
different in each of these cases, the general 
behavior of the ear, as analyzed in this volume, 
will be the same. 

In Part I, comprising Chapters 1 and 2 of 
this volume, are presented the relevant facts 


on the structure and performance of the ear as 
revealed primarily by published psychoacousti- 
cal studies before World War II. Part II, con- 
sisting of Chapters 3 through 6, analyzes the 
data on the masking of sounds produced by a 
distant target, while Chapters 7 through 11 
which constitute Part III deal with the cor- 
responding results for echoes. For convenient 
practical application, the chief results found in 
these two parts are briefly stated in Chapters 
6 and 11. Most of the material presented in 
Parts II and III was obtained during the war, 
in part by the British, and in part by the three 
following groups under contract with Division 6 
of the National Defense Research Committee: 
the University of California Division of War 
Research at the U. S. Navy Electronics Labora- 
tory (formerly the U. S. Navy Radio and Sound 
Laboratory), San Diego, California; Columbia 
University Division of War Research at the 
U. S. Navy Underwater Sound Laboratory, New 
London, Connecticut; and Bell Telephone Lab- 
oratories, New York, New York. 

This volume has been written by Dr. Aaron 
Spector, who has introduced much analytical 
discussion which is not available elsewhere. 
Final preparation and editing of the material 
has been carried out cooperatively by the staff 
of the Sonar Analysis Group. The laboratory 
groups whose work is reported here have been 
extremely helpful in communicating results in 
advance of publication and in discussing the 
many problems of interest in this subject. Par- 
ticular acknowledgement is due to the Uni- 
versity of California Division of War Research, 
and to Bell Telephone Laboratories. 

Lyman Spitzer, Jr. 


CONTENTS 


PART 1 

THE EAR AHD HEARING 

CHAPTER PAGE 

1 Structure of the Ear 1 

2 Behavior of the Ear 12 

PART II 
LISTENING 

3 Characteristics of Target Sounds and Noise 

Background 37 

4 Laboratory Measurements on Masking of Tar- 
get Sounds 54 

5 Field Measurements on Masking of Target 

Sounds 134 

6 Summary of Listening Studies 153 

PART III 
ECHO RANGING 

7 Characteristics of Echoes and Reverberation 156 

8 Noise Masking of Echoes 170 

9 Reverberation Masking of Signals without 

Doppler 209 

10 Reverberation Masking of Signals with Doppler 234 

11 Summary of Echo Studies 260 

Bibliography 265 

Contract Numbers 272 

Index 273 


/ IkKS I HICI Kl) ^ xi 



Chapter 1 

STRUCTURE OF THE EAR 


F undamental to any discussion of the ear 
and hearing are references 1 through 5. 
These works, and the literature cited in them, 
are the sources for all statements about the 
structure and behavior of the ear which are 
made in this report and which are not specifi- 
cally attributed to some other source. A few 
extensions and applications of the fundamental 
concepts have been prepared especially for the 
present report. These are relatively minor and 
are not identified. Although an effort has been 
made to specify the limitations of present 
knowledge, it is clearly impossible to discuss 
all the details in a condensed summary of this 
kind. Careful reading of the original publica- 
tions is necessary for a thorough understand- 
ing of this subject. 



OUTER EAR- 


MIDDLE-^^ 

EAR 


INNER £AR 



Figure 1. Schematic diagram of the ear. 


A few standard results of physical acoustics 
are assumed without proof in this volume. For 
a discussion of these, the reader is referred to 
references 6 and 7. 


The potentialities of the ear as a detector 
depend on its nature as a physical mechanism. 
A brief account of present knowledge about the 
structure and behavior of the ear is given in 
the following two chapters and the concepts 
developed there are applied in the remaining 
chapters of this volume to the data obtained in 
studies of sound gear. 



Figure 2. Schematic diagram of the outer, mid- 
dle, and inner ear. 


The ear structures are minute and delicate. 
Few of them are accessible to direct observa- 
tion when the ear is intact.® Most of our ideas 
about the hearing process are consequently 
based on less direct lines of evidence. Some 
of the more important features of the normal 
human ear are illustrated schematically in Fig- 
ures 1 through 8. These structures are believed 
to behave in the following way during the act 
of hearing. Audible sound waves which enter 
the external ear and pass through the ear canal 
strike the drum membrane (see Figure 1) . The 
resulting motion of this membrane is transmit- 
ted by the ossicles, a fiexibly jointed chain of 
three small bones stretching across the middle 
ear, to the inner ear which is the essential organ 
of hearing."" The nerve impulses generated in 

a Destruction of the outer ear, or of the ossicles, in- 
terferes with hearing; destruction of the inner ear or 
of the auditory nerve produces total deafness. 


1 


2 


STRUCTURE OF THE EAR 


the inner ear move along the auditory nerve to 
the brain, where they produce the sensations 
of hearing. 

Present evidence indicates that perceptions 
of pitch which are aroused by stimulating the 
ear with tones of different frequency are cor- 
related primarily with positions of stimulation 
pattern on the sensory membranes of the inner 
ear. This point of view is generally known as 
the place theory. Its adequacy for frequencies 
below 100 cycles has not been established. The 
fact that a particular place on the sensory mem- 
brane is stimulated by a given tone is consid- 
ered to result from the selective tuning of the 
membrane, so that different tonal frequencies 
produce vibration patterns on this membrane 
which have their maximum amplitudes at dif- 
ferent places. The latter explanation of the 
ear’s behavior is called the resonance theory. 
The place theory and the resonance theory are 
supplementary; taken together, they provide a 
consistent understanding of the bulk of the 
known facts. Continued study may require 
modification of these points of view, which are 
nonetheless useful at present in organizing the 
data and in suggesting further experimental 
work. The significance and the application of 
these theories can best be understood from an 
examination of the data regarding the inner 
ear structures; these data are presented in 
the following section. 


INNER EAR 

The inner ear structures are contained in a 
coiled bony tube, named the cochlea from its 



Figure 3. Section through the cochlea. 

resemblance to a small snail shell. To shield 
the inner ear from injury and to insulate it 
partly from the effects of sounds which are 


not directed to enter it through the external 
ear canal, the cochlea is enclosed within the 
wall of the skull. The compact form of the 
cochlea probably results in greater mechanical 
strength. 

A sectional view of the cochlea is shown in 
Figure 3. Figures 4 and 5 show how it would 
look if unrolled. It is some 31 millimeters in 
length, wider at its base than at its apex, par- 
titioned along its length by the sensory mem- 


BASAL END 
(HIGH FREQUENCIES) 


APICAL END 
(LOW FREQUENCIES) 


OVAL 

WINDOW 


ROUND 

WINDOW 


UPPER GALLERY (mean depth -O.Bmm) 

LOWER GALLERY (meon depth = 0.8 mm) 




Figure 4. Sectional view of the basilar mem- 
brane, oval window, and the round window. 


branes, and completely filled with clear watery 
fluid. The walls of the cochlear duct are rigid 
except for two openings at its base called the 
oval ivindotv and the round ivmdoiv, from their 
shapes. These windows adjoin each other on 
the surface of the middle ear cavity. The round 
window is sealed by a thin elastic membrane. 
The oval window has one end of the chain of 
ossicles secured to its margins by means of tis- 


8ASAL END APICAL END 

(HIGH FREQUENCIES) (LOW FREQUENCIES) 



Figure 5. Sectional view of the basilar mem- 
brane, bony ledge, and the spiral ligament. 


sues, in such a manner that motion of the 
ossicles is relatively free, although the inner 
ear fluid cannot escape; the other end of the 
chain of ossicles is attached to the drum mem- 
brane. Increase of pressure at the drum dis- 
places it and the ossicles inward, shifting the 
inner ear fluid and causing the membrane at 
the round window to bulge. This motion of the 
cochlear fluid stimulates the auditory nerve. 

As shown in Figures 1 and 2, the cochlea 
is part of a closed system of communicating 
tubes, the so-called labyrinth, which contains 


RESONANCE THEORY OF HEARING 


3 


several sense organs and is filled with liquid. 
Since the walls of the labyrinth are relatively 
rigid, except for the round and oval windows, 
motion of the middle ear structures causes mass 
displacement of fiuid within the cochlea only. 
The structures of the labyrinth other than the 
cochlea are not described here because they are 
concerned with bodily equilibrium and not with 
hearing. 

The cochlear canal resembles a spiral stair- 
case wound around a bony central shaft (see 
Figure 3). This canal is incompletely divided 
along its length by a bony ledge; the partition 
of the canal into an upper and a lower gallery 
is completed by a thin membrane (the basilar 
membrane) in which are embedded throughout 
its length a great number of fine fibers (the 
auditory strings). The auditory strings lie 
parallel to the diameter of the cochlear canal 
and are maintained in tension by the spiral 
ligament. The basilar membrane does not ex- 
tend quite to the apex of the cochlea (see Fig- 
ures 4 and 5) ; the edges of the membrane, with 
the exception of its apical edge, are fastened 
to the walls of the cochlear duct. Thus, fluid 
may pass from one gallery into the other only 
through the gap at the apex of the basilar 
membrane. 

Since the round and oval windows lie on op- 
posite sides of the basilar membrane, the pis- 
ton motion of the ossicle at the oval window 
will produce displacement of the cochlear fluid, 
either by causing it to flow through the gap 
at the extreme end of the basilar membrane, 
or by moving it along a hydraulic short circuit 


BASAL END APICAL END 

(HIGH FREQUENCIES) (LOW FREQUENCIES) 



Figure 6. Assumed motion of the inner ear 
structures in response to sound. 


(illustrated in Figure 6) which is accompa- 
nied by bulging of the basilar membrane. The 
actual hydraulic path, according to the reso- 


nance theory, depends upon the impressed fre- 
quency. 


12 RESONANCE THEORY OF HEARING 

It is assumed that the basilar membrane is 
selectively tuned in the manner of a reed-type 
frequency meter so that different portions of 
the membrane will vibrate in synchronism with 
those impressed frequencies to which they reso- 
nate. The possibility that sensations of pitch 
are associated with the wavelengths of the re- 
ceived sounds may be excluded, since the dimen- 
sions of the cochlea are so small compared with 
the wavelengths of audible sounds. Also, ob- 
servation indicates that the auditory nerve and 
brain do not directly analyze the frequency of 
a stimulus. For example, direct stimulation of 
the auditory nerve with alternating current of 
any frequency invariably produces the sensa- 
tion of pitchless noise. 

In the present section, the physical charac- 
teristics of the resonant mechanisms of the 
inner ear will be examined. Assuming that 
the dynamical behavior of the basilar mem- 
brane may be satisfactorily approximated by 
considering it as an assembly of loosely coupled 
strings, an arrangement resembling a Venetian 
blind, it follows that the region of the mem- 
brane which responds best to a particular fre- 
quency is determined by the combined effect 
of several factors: the lengths of the auditory 
strings 1; their tensions T ; and their mass per 
unit length m, including the effective increase 
of mass due to the column of fluid which must 
be moved when some portion of the membrane 
moves. 

In the following paragraphs the dependence 
of the resonant frequency on these physical 
properties of the inner ear is examined. Al- 
though the available data do not yield precise 
values of the relevant quantities, the investiga- 
tion indicates that resonance of the basilar 
membrane for perceived sonic frequencies is 
not unreasonable physically. Moreover, this 
analysis yields a useful if approximate expres- 
sion for the position of maximum stimulation 
on the membrane, for sound of a particular 
frequency. 


4 


STRUCTURE OF THE EAR 


The basic expression for the resonant fre- 
quency / of a string under a tension T dynes, 
of length I, and with a mass m per unit length 
is 

f=Ul- 

This ignores the effects of damping, which 
would diminish the value of the resonance fre- 
quency somewhat. Such damping would be 
expected from the effects of friction and vis- 
cosity in the various parts of the ear. Damp- 
ing, together with the fact that adjoining audi- 
tory strings are coupled by the ground sub- 
stance of the basilar membrane, should result 
in a broadened, rather than a sharply defined, 
response; there is evidence that this occurs, 
particularly for large amplitudes (see Figures 
2 and 3 of Chapter 2) . The advantage of damp- 
ing is that sensations of sound are extinguished 
soon after the stimulus is removed. 

The relation between the position s of a 
string on the basilar membrane and the fre- 
quency to which it responds may be estimated 
from the mechanical constants of the inner ear 
(see Figures 4, 5, and 6). It seems reasonable 
to infer from these constants that the apical 
end of the membrane is tuned to low frequen- 
cies; and the basal end, to high. The auditory 
strings at the apical end, for example, are ap- 
proximately five times as long as those at the 
basal end ; the width of the membrane decreas- 
ing fairly uniformly from about 0.50 milli- 
meter at the apex to about 0.10 millimeter at 
the base. The length, in centimeters, of an 
auditory string may therefore be represented 
as follows: 

I = (5.0 X 10-2) - (1.3 X 10-2) s, (2) 

where s is the distance of the string from the 
apex of the basilar membrane in centimeters; 
thus, at the base of the membrane, s equals 
3.1 centimeters. 

The tension exerted by the spiral ligament 
on the shorter basal strings is believed to ex- 
ceed that exerted on the apical strings since the 
ligament is not a uniform structure. It consists 
of a few frail fibers at the apical end and thick- 
ens gradually toward the basal end of the mem- 
brane. The simplest relation to try, in the 
absence of more definite information, is one 


which makes the tension increase linearly with 
distance from the apex, that is, 

T = as, (3) 

where s is subject to the same restriction as 
before and a is a constant. Equation (3) is, 
of course, somewhat arbitrary; lack of infor- 
mation concerning T is perhaps the greatest 
gap in our knowledge of the physical parame- 
ters affecting the resonance of the inner ear. 

The effective mass of an auditory string de- 
pends on the liquid loading and may be taken 
as proportional to the mass of the moving col- 
umn of fluid. The mass of piano strings is in- 
creased in similar fashion, without adding sig- 
nificantly to their rigidity, by coiling copper 
wire on them. As indicated in Figure 6, the 
length of such liquid columns is very nearly 
2 (3.1 — s) , and the expression for the mass per 
unit length of a string may therefore be written 

m = 6 (3.1 - s), (4) 

where h is assumed here to be a constant, 
depending on the density of the cochlear fluid. 
As shown in the next section, there is some 
reason to believe that h may vary somewhat 
along the membrane. In view of the lack of 
specific information about T, this complication 
is not considered here. 

If the motion of the ossicle at the oval win- 
dow is sufficiently slow, pressure above and 
below the membrane will remain equal. Hence, 
it will not be displaced from its equilibrium 
position along its length, and cochlear fluid will 
pass from one gallery to the other through the 
gap at the apex of the membrane, producing 
simultaneous displacement of the round win- 
dow. For sufficiently rapid displacements of 
the ossicle, or for high enough frequencies of 
vibration, the cochlear fluid has considerable 
impedance. Hence, when the ossicle thrusts 
inward, the pressure in the upper gallery will 
exceed that in the lower, and the membrane 
will be displaced as shown in Figure 6. Those 
portions of the membrane with natural fre- 
quencies equal, or nearly equal, to the im- 
pressed frequency will move with greater am- 
plitudes since their motions can remain in 
phase with the driving force. While general 
forced motion of the entire membrane is to 
be expected, especially for sounds of high in- 


RESONANCE THEORY OF HEARING 


5 


tensity, relatively small amplitudes should be 
found at all points whose natural frequency 
differs from that of the stimulus tone. 

By substituting equations (2), (3), and (4) 
into equation (1), and denoting the fractional 
distance from the apex to the base of the mem- 
brane, that is, s/3.1, by the letter x, we obtain 

lOVa/b I X _ 1300 / X _ . . 

The constant has the dimensions centi- 

meters per second. A numerical value of 1,300 
has been assigned to this quantity to give the 
best agreement with the observed data (see 
Figures 9, 10, and 17 in Chapter 2). Since 
equation (5) has been derived from several 
quite approximate assumptions, it cannot be 
regarded as rigorous. However, it should give 
results correct to within an order of magni- 
tude. Comparison with the data in Section 2.2 
and 2.3 shows that this equation has, in fact, 
the proper form. It may therefore be regarded 
as a semi-empirical relationship which provides 
a useful summary of a large number of obser- 
vations. For example, when x increases by 
equal increments, / grows by successively larger 
amounts. In other words, the positions on the 
basilar membrane corresponding to two fre- 
quencies which differ by a constant number of 
cycles are closer together for high frequencies 
than for low. Furthermore, when x is 0.5, / 
equals 2,200 cycles per second, that is, the cen- 
ter of the basilar membrane is tuned to a fre- 
quency a little over 2 kilocycles. Both these 
conclusions are in agreement with observation 
(see Section 2.2.1) . 

The resonance theory of hearing thus pro- 
vides a correlation, given by equation (5), be- 
tween the position of an auditory string and 
its resonance frequency. This prediction is in 
good agreement with observation. However, if 
this theory of the ear’s structure is to be re- 
garded as creditable, it must provide acceptable 
answers to several other questions. 

For example, are the estimated tensions in 
the auditory strings of reasonable magnitudes? 
Is the required rate of change of tension be- 
tween neighboring strings immoderately large? 
If certain simplifying assumptions are made, 
orders of magnitude may be computed for these 


two quantities. The computations are given in 
the following subsection. 


^ Tension in Basilar Membrane 

Solving equation (1), we obtain 

T = ApPm. (6) 

If the required tension is unduly large, this 
fact would show up most clearly at the higher 
frequencies. It will be assumed, therefore, that 
/ equals 10 kilocycles, which is within an octave 
or so of the highest audible frequencies. From 
equation (5) it follows that x equals 0.85 for 
a frequency of about 10 kilocycles ; hence s, the 
position of the string, is about 2.6 centimeters 
from the apex of the membrane, or about 0.5 
centimeter from the oval window. The length 
of a string at this position is, from equation 
(2), approximately 1.6 X 10-^ centimeter. 

In addition, m must be evaluated ; this quan- 
tity is the mass M of the liquid column loading 
the string, divided by the length I of the string. 
The evaluation of m is not a simple matter 
but depends on the solution of the hydrody- 
namical equations, taking into account the 
physical properties both of the cochlear canal 
and of the basilar membrane. An upper limit 
may be derived by assuming that all the fluid 
in the cochlear canal between the oval window 
and vibrating section of the membrane vibrates 
to and fro. This neglects the effect of viscosity 
and also neglects the possibility that at the 
higher frequencies the wavelength of sound in 
the cochlear canal may be less than the total 
length of the canal. On this simple assumption, 
the total volume of the oscillating fluid is 
2A(3.1 — s), where A is the cross section of 
each of the two canals, and (3.1 — s) is the 
length in centimeters of the vibrating column. 
The factor 2 takes into account the fact that 
the liquid in both canals takes part in the oscil- 
lation. To find the total mass, this volume must 
be multiplied by the density p of the cochlear 
fluid. This density is 1.034 grams per cubic 
centimeter at body temperature and may be 
set equal to unity for these calculations. 

This oscillating mass has a cross-sectional 
area A which is considerably greater than the 


O f RESTRICTED I 


6 


STRUCTURE OF THE EAR 


area lAs of the vibrating region of the mem- 
brane (see Figure 6). As a result, the maxi- 
mum fluid velocity is less than the maximum 
velocity of the membrane by a factor Ias/A. 
Since the mass is important through its con- 
tribution to the kinetic energy, the effective 
mass is roughly equal to the oscillating fluid 
mass multiplied by (Ias/A)^. This rather sim- 
ple approach is, of course, not rigorous but 
gives the same general result as a more de- 
tailed analysis of the potential and kinetic en- 
ergies of the system. 

To And the mass loading per string, the total 
vibrating mass must be divided by the number 
of strings which are set vibrating by a pure 
tone. This number is about equal to 100 for 
a tone of moderate loudness (see ‘‘Beats’’ in 
Section 2.1.2). Thus we have Anally 

M 2 4 (3.1 -s) HasT 

“ = T = LtJ • 

If we substitute equation (7) into (6), let 
A equal 6 X 10"^ square centimeter, and As equal 
3 X 10-2 centimeter (see “Beats” in Section 
2.1.2.), we find at a frequency of 10 kilocycles, 

_ SpP(Aspi3.1 - s) 

100 A 

_ 8(10») (1.6 X 10-2)3 (3 X 10-2)2 (5 X lO"') 

102 X 6 X 10-^ 

= 2.4 dynes 

corresponding to a force of about 2.5 milli- 
grams. Since the cross section of a single 
string in the membrane is about 10-^ square 
centimeter, the total stress is 2.5 X 10^ dynes 
per square centimeter, or about 0.17 ton per 
square inch. 

This result may be compared with the break- 
ing strength of spider thread, 15 tons per 
square inch, and of silk, 32 tons per square 
inch. Since equation (7) gives an upper limit 
for m, the stress found from the resonance 
theory appears very plausible. Furthermore, 
the rate at which the estimated tension changes 
along the length of the membrane, in other 
words, the quantity a in equation (3), is fairly 
small, totaling about 1 dyne per millimeter (25 
dynes per 26 millimeters). 


^ Hydrodynamic Theory 

For the sake of simplicity the discussion here 
has been built around the most elementary 
form of the resonance theory. A more general 
treatment will be found in reference 9. Con- 
sideration is given there to the hydrodynamic 
equations appropriate to the situation in which 
a pressure pulse is initiated at one end (the 
oval window) of a liquid-filled tube with three 
rigid walls and one flexible wall, corresponding 
to the bony structure of the cochlear duct and 
the yielding structure of the basilar membrane. 
The analysis indicates that the pressure gradi- 
ents associated with successive cycles of a 
stimulus tone are propagated along such a tube 
with a speed of about 50 m per sec. This low 
velocity arises from various viscous and fric- 
tional factors and is in fair agreement with 
the velocity of approximately 20 meters per 
second derived from the functional response 
of the ear. Furthermore, this theory indicates 
that the amplitudes of the traveling wave will 
be greatest at specific points of the basilar 
membrane, and that the positions of these 
points are associated with the frequencies of 
the impressed tones. The agreement between 
the predicted and observed positions is inferior 
to that obtained with equation (5) which may 
be due to somewhat uncertain knowledge of the 
constants needed for the development of the 
analysis and also to the effects of various sim- 
plifying assumptions. 

The time required for a disturbance, moving 
at a velocity of 2,000 centimeters per sec, to 
pass over the distance of 3.1 centimeters be- 
tween the oval window and the apex of the 
membrane is about 1.5 milliseconds. Hence, 
the high-frequency content of a complex acous- 
tic disturbance made up of all frequencies is 
sensed about 1.5 milliseconds earlier than the 
low-frequency content. 

The end organs of equilibrium are essentially 
plumb-bobs suspended in the fluid of the semi- 
circular canals. When these are displaced, a 
sense of disequilibrium is produced. Pressure 
variations in the cochlear fluid are transmitted 
to the semicircular canals, since the cochlea is 
but one of a group of communicating tubes 
composing the labyrinth. Ordinarily, pressure 


SENSORY AND NERVOUS STRUCTURES 


7 


changes in the cochlea, produced by incident 
sounds, cause no motion of the equilibrium re- 
ceptors in the semicircular canals, because the 
pressures at any point are independent of di- 
rection. This ceases to be true in the case of 
rotational, or vortex, motion; and it has been 
pointed out^° that vertigo and reflex motion of 
the head in response to very loud sounds is 
probably associated with the production of 
eddies in the cochlea. 


13 SENSORY AND NERVOUS 

STRUCTURES 

Thus far we have spoken of the basilar mem- 
brane as though it alone were responsible for 
the perception of tones. Actually, the motion 
of the membrane which is produced by tonal 
stimulation causes a localized disturbance of 
sensory cells attached to the upper surface of 
the membrane, and this disturbance of a par- 
ticular group of sensory cells causes speciflc 
flbers in the auditory nerve to respond. 

The arrangement of the sensory structures 
supported by the basilar membrane is shown in 
Figures 7 and 8. A sequence of about six thou- 
sand adjoining arches projects from the sur- 
face of the basilar membrane into the upper 
gallery (Figure 4). These are called Corti 
arches after their discoverer. They are uni- 
formly spaced over the entire length of the 
basilar membrane. Each of the arches is 
formed from two stiff bristles which are joined 



Figure 7. View of the basilar membrane through 
the surface of the upper gallery, showing the 
Corti arches. 

at their upper ends to form a fairly rigid truss. 
One leg of each arch is supported on the mar- 
gin of the bony ledge; the other rests on the 
basilar membrane. The base of an arch mea- 
sures about a quarter the width of the mem- 


brane; in other words, the arches increase in 
size along with the auditory strings. Thus, as 
the auditory strings vibrate, the arches rock 
back and forth, pivoting upon the leg which 
rests on the bony ledge. The exact function 



Figure 8. View of the basilar membrane show- 
ing the Corti arches and hair cells. 


which the Corti arches play in the mechanism 
of hearing is not certain. 

As shown in Figure 8, each of the arches is 
flanked by four sensory cells. These cells form 
two rows running the length of the basilar 
membrane. The outer row, which rests on the 
basilar membrane, is three cells deep ; the inner 
row, which rests on the bony ledge, is only one 
cell deep. These sensory cells are called hair 
cells because of the tiny hairs which project 
from their upper surfaces. The hair cells are 
the end organs of hearing; that is, the fibers 
of the auditory nerve terminate in the hair cells 
(see Figures 3 and 8). 

Thus, the auditory nerve resembles a tele- 
phone cable supplying a distribution board with 
about 25,000 terminals (6,000 arches with four 
hair cells per arch). The nerve trunk enters 
the cochlea through the bony shaft around 
which the cochlear duct is coiled. Its individual 
fibers branch out to the hair cells through the 
bony ledge of the cochlear canal (see Figures 
3 and 8) . 

All the hairlets which project from the free 
ends of the hair cells are embedded, as shown 
in Figure 8, in a soft structureless membrane 
(the tectorial membrane) lying above the hair 
cells. One margin of the tectorial membrane is 
secured to the bony ledge; the other margin is 
free. The hairlets, being embedded in the tec- 
torial membrane, cannot follow the rocking 



8 


STRUCTURE OF THE EAR 


motion of the hair cells and Corti arches which 
is produced by vibration of the auditory 
strings ; instead they are bent to a degree which 
depends upon the amplitude of motion of the 
basilar membrane. It is believed that this bend- 
ing of the hairlets and possibly the compres- 
sions and elongations which they suffer stimu- 
late the fibers of the auditory nerve. Thus 
“hearing” may be compared to a highly differ- 
entiated sense of touch. 

This description of the action of the hair 
cells is in agreement with several experimental 
observations. When the row of three outer hair 
cells is congenitally missing (in animals with 
hearing apparatus similar to that of man) and 
the row of inner hair cells is intact, the animal 
is partly deaf. In such cases it is necessary to 
raise the level of a tone about 30 to 40 decibels 
above the normal threshold of audibility before 
that tone can be heard. This implies that the 
row of three outer hair cells responds to faint 
sounds and that the inner hair cells respond 
only to loud sounds. If the hair cells are thought 
of as mounted upon a lever with its fulcrum 
located on the margin of the bony ledge (see 
Figure 8) it will be observed that the three 
outer hair cells, which are situated on the long 
arm of the lever, will undergo a larger displace- 
ment than will the inner hair cells, for any 
given amplitude of motion of the basilar mem- 
brane. 

The converse observation to that described 
above is the following. By continued exposure 
to loud tones, animals with inner ear structures 
like that of man can be partly, but permanently, 
deafened to the tones to which they are exposed. 
The extent of this deafening never exceeds 30 
to 40 decibels ; that is, the experimental animals 
can always hear such tones subsequent to ex- 
posure if the intensity level is raised the stated 
amount above normal threshold. In other words, 
the receptors for the fainter sounds are distinct 
from those which respond only to loud sounds. 


1 ^ ELECTRICAL ACTIVITY 

Two methods are commonly used to deter- 
mine the sensitivity of animals to sound. In the 
first place, they may be conditioned to associ- 
ate some pleasant or painful experience with a 


particular sound which invariably accompanies 
that experience in the experimental situation. 
After repeated association of this kind, the test 
animal gives the same observable response, 
such as flight or salivation, to the sound that it 
would normally give to the experience with 
which the sound is connected. By diminishing 
the intensity of the test sound until no response 
to sound is obtained, the auditory threshold of 
the experimental animal can be determined. 

The second method is more direct and pro- 
vides some results which are probably not ob- 
tainable through use of the first. It is based on 
the observation that stimulation of the ear by 
sound produces a measurable change of elec- 
trical potential at the cochlea. These cochlear 
potentials may be detected by placing one elec- 
trode on the surface of the head and the other 
in contact with the cochlea, after it* has been 
exposed surgically. These changes of potential 
are of the order of microvolts, and amplification 
is required for detailed study. The cochlear 
potentials are strongest when one electrode 
makes contact with the cochlea but they can 
be detected almost anywhere on the head if 
sufficient amplification is used. Wave form, and 
changes of wave form or intensity, of sound 
waves which are used to stimulate the ear are 
very faithfully reproduced by the cochlear po- 
tentials. Thus, speech entering the ear of an 
experimental animal is intelligibly reproduced 
by an amplifier and loudspeaker circuit which 
is actuated by the cochlear potentials. Standard 
indicating instruments for such studies are the 
cathode-ray screen and the wavemeter. The 
cochlear potentials obviously furnish a very 
powerful tool for investigating such effects as 
the change of auditory acuity which is produced 
by long exposure to loud tones. 

When an alternating current is passed 
through the head under appropriate conditions, 
sensations of tone are produced whose pitch 
corresponds to the frequency of the current. 
This phenomenon is termed the electrophonic 
effect. Formally, at least, it is the inverse of 
the cochlear potentials which are produced by 
conversion of mechanical energy into electrical. 

Several other matters related to the cochlear 
potentials are worth discussing at this point. 
To begin with, the cochlear potentials must be 


lESTRICTEfe 


3 ^ 


OUTER AND MIDDLE EAR 


9 


distinguished from the potentials associated 
with the activity of the fibers in the auditory 
nerve. When a nerve fiber, including the audi- 
tory nerve fibers, responds to a stimulus, this 
action is accompanied by the development of 
a potential along the fiber which is never ob- 
served in the quiescent state of the nerve. Such 
potentials are therefore called the action po- 
tentials of the nerve. When the auditory nerve 
is severed from the cochlea, the action poten- 
tials can no longer be initiated by means of 
sound. However, chemical or mechanical stimu- 
lation may be substituted. Under these condi- 
tions, action potentials and cochlear potentials 
continue to be observed independently of each 
other. In other words, the auditory nerve is not 
essentially a passive cable which transmits the 
cochlear potentials to the brain; its role in the 
hearing process can be understood only in 
terms of its behavior as a nerve. 

Nerve fibers, including the fibers of the audi- 
tory nerve, respond to chemical, mechanical, 
and electrical stimulation. They do so only 
when the stimulation exceeds a threshold value. 
This threshold differs from nerve to nerve, it is 
different for the various fibers of a given nerve, 
and it depends upon the previous stimulation 
which a given fiber has received. Furthermore, 
the strength of the action potential which de- 
velops when the response threshold of a nerve 
is exceeded is independent of stimulus inten- 
sity; in other words, a nerve discharge either 
occurs at full strength or it does not occur at 
all. Nerve response is therefore “all or none’’ 
in character and is frequently compared to the 
firing of a gun. Nerve discharge resembles gun- 
fire in one other respect; the fiber needs time 
to “reload,” or, more exactly, it enters a re- 
fractory phase after discharge and cannot dis- 
charge again until it recovers its sensitivity. 
The duration of this refractory period is about 
1 millisecond (10"'^ second) ; hence the maxi- 
mum rate of discharge of any nerve fiber is 
about one thousand per second. The cochlear 
potentials, on the other hand, are continuous 
functions of the stimulus and can follow the 
latter to very high frequencies. 

The origin of the cochlear potentials and the 
part they play in hearing are uncertain at the 
present time. However, they mirror the me- 


chanical events occurring in the cochlea ac- 
curately and are therefore useful in studying 
these events. For example, comparison of action 
with cochlear potentials shows that the nerve 
fibers which are stimulated by a given tone fire 
in synchronism and that they are excited during 
the rarefaction of the sound wave, that is, when 
the basilar membrane moves up against the 
tectorial membrane. 


» 5 OUTER AND MIDDLE EAR 

The foregoing discussion of the structure of 
the ear has emphasized the inner ear, since this 
is the organ where different frequencies are 
distinguished and where mechanical oscillations 
are converted into nerve impulses. For a better 
understanding of the auditory process, how- 
ever, information on the other parts of the ear 
is also important. The structure and behavior 
of the outer and middle ear are therefore dis- 
cussed in this section. 


^ Outer Ear 

The outer ear acts as a collector of sound. 
It does so mostly by scattering energy into the 
external ear canal, since its dimensions are not 
large enough, relative to the wavelengths of 
audible sounds, to permit efficient reflection. 
Since scattering increases with increasing fre- 
quency, the external ear is a more efficient col- 
lector for high-frequency sounds than for low- 
frequency sounds, and, because of its shape and 
position, is a more efficient collector for high- 
frequency sounds directed toward the face than 
it is for those directed toward the back of the 
head. 

This fact can be demonstrated readily with 
the aid of a high-quality watch. The tick of 
such a watch is rich in high-frequency sound. 
If the watch is held beside the head, it will 
sound louder when in front than it does when 
behind the ear. This effect can be augmented 
by holding the cupped hand, concave forward, 
near the ear. In this case there is a considerable 
increase in loudness, when the watch is held in 
front, at the instant that the edge of the cupped 


G 


RESTRICTED 


9 


10 


STRUCTURE OF THE EAR 


hand is brought into contact with the ear. This 
is a resonance effect and is similar in origin to 
the roaring sounds heard when a seashell is 
held to the ear. 

In other words, resonances in the external 
ear can amplify the loudness of some sounds or 
render them audible when they would otherwise 
be too faint to hear. Thus, measurements have 
been made^^ at a fixed position in space, later to 
be occupied by the head of the subject, of the 
sound pressure produced by a standard source. 
These measurements were repeated within the 
auditory canal when the subject’s head was 
brought to the test position. The sound pickup 
was a narrow flexible tube whose free end could 
be inserted into the ear canal. The other end of 
this probe tube was used to drive a condenser 
microphone. For tones between 1 and 5 kilo- 
cycles, the pressure within the ear canal ex- 
ceeded the free-held pressure. The greatest 
amplification, for tones with frequencies of 2 to 
3 kilocycles, was between 9 and 15 decibels. 

These measurements have recently been re- 
peated in an echo-free chamber,^^ ^nd differ- 
ences between the free-held and auditory canal 
pressures as large as 20 decibels are reported 
for the case of 3-kilocycle tones. The resonance 
frequency and degree of amplihcation are found 
to vary from subject to subject and probably 
depend upon the shape and the size of the ex- 
ternal ear. In other words, the ear canal 
behaves as though it were a closed tube with 
rigid walls and a length of about 2 centimeters. 
For comparison, the average depth of the ear 
canal is about 2.7 centimeters, and its diameter 
about 0.7 centimeter. Furthermore, it has been 
found^^ that the resonance frequency of the ear 
canal may be changed by hlling it with hydro- 
gen or coal gas. These observations lend sup- 
port to the view^^ that reduction of deafness 
following surgery is often an incidental by- 
product and is largely due to a change in the 
resonance frequency of the outer ear, thereby 
improving response in some frequency regions 
and diminishing it in others. 

^ Middle Ear 

The ear drum is shaped somewhat like a 
loudspeaker cone and has very little inertia. 


It normally seals off the middle ear, which is 
essentially an air-filled cavity, from the ex- 
ternal atmosphere. Differences of static pres- 
sure across the drum (such as are produced 
by rapid changes of altitude) constrain its mo- 
tion and affect the responsiveness of the ear to 
sound. 

The middle ear communicates with the throat 
through the Eustachian tube (see Figures 1 
and 2) . This tube opens during the act of swal- 
lowing, or yawning and thereby equalizes the 
pressures acting on the drum membrane. For a 
discussion of injuries produced by failure to 
maintain equality of middle ear and external 
pressures, in the case of submariners making 
practice escapes from a depth of 100 feet below 
the surface, see reference 15. 

If the pressures exerted upon the round and 
oval windows were equal, they would cancel 
each other’s effects, and motion of the basilar 
membrane would not occur. These pressures 
are unequal, however, because the force acting 
on the drum is transmitted to the oval window 
through the ossicles and to the round window 
by an air path. Since the drum has an area 
about twenty-five times that of the oval window, 
and since the ossicles are equivalent to a lever 
with a mechanical advantage of about 1.5, the 
pressure at the oval window is approximately 
forty times greater than at the drum. Similarly, 
the pressure at the drum is probably about ten 
times higher, for a 2-kilocycle tone, than that 
in the incident sound wave, owing to resonance 
amplification in the ear canal. 

Since the motional impedance of air is less 
than that of the inner ear fluid, the middle and 
outer ears act as impedance matching devices. 
Measurements of the overall impedance of the 
ear show that the match to that of air is good 
in the region between 800 and 1,500 cycles per 
second. 

Several mechanisms in the ear reduce some- 
what the amplitude of response to loud sounds. 
In the first place, the ossicles cease to move as 
a unit when high-intensity sounds reach the 
ear; and for large displacements of the ear 
drum there is considerable sliding and friction 
at the joints in the chain of ossicles. Nonethe- 
less, the amplitude of motion of the ossicles 
continues to increase with intensity. 


mESTRICTE 


OUTER AND MIDDLE EAR 


11 


In the second place, the ossicles are con- 
trolled, as shown by the arrows in Figure 2, by 
the action of two antagonistic muscles. These 
are tensed by reflex action in the presence of 
loud sounds and thereby diminish the amplitude 
of motion. Measurement shows that the pro- 
tective action of the middle ear muscles is con- 
fined to frequencies below 1 kilocycle. 

The middle ear mechanism becomes progres- 
sively less responsive to increasingly intense 
sounds. Indeed, the ossicles may disarticulate 
in the vicinity of the pain threshold. The tilts 
and relative positions of the ossicles tend to 
change as the amplitude of their motion in- 
creases. Thus, the mechanism of the middle ear 
is nonlinear, as illustrated in Figure 9. Here, 
the heavy curve represents the transfer charac- 
teristic of the middle ear. It will be seen that 
the transmitted displacements do not keep pace 
with the applied displacements. 

For the sinusoidal input wave sketched on 
the vertical axis, the transmitted disturbance 
has the form shown on the horizontal axis. The 
output wave is seen to be flattened, with respect 
to the input, but the positive and negative half 
cycles are symmetrical about the horizontal 



Figure 9. Nonlinear transfer characteristic. 


line. This distortion of wave form, due to non- 
linearity of the transfer characteristic, intro- 
duces higher frequencies into the transmitted 
disturbance which are odd harmonics (odd in- 


tegral multiples) of the input frequency and 
which are not present in the input. 

When the transfer characteristic is asym- 
metrical, as well as nonlinear (see Figure 10), 



Figure 10. Asymmetrical and nonlinear trans- 
fer characteristic. 


the transmitted wave contains the even har- 
monics of the fundamental input frequency in 
addition to its odd harmonics. Observations 
cited under “Aural Harmonics’" in Section 2.1.2, 
indicate that the ear generates both the odd and 
even harmonics of single-frequency sounds in- 
cident upon it. 

Although the ligaments from which the 
ossicles are suspended in the middle ear cavity 
generally prevent lateral vibrations of this 
chain of bones for low-intensity sounds, this is 
probably no longer true for large displacements 
and introduces a further source of distortion. 
It should be pointed out, in conclusion, that the 
middle ear is probably not the only locus of dis- 
tortion. Thus, the ear drum very likely vibrates 
in segments, when the incident intensity is 
high, instead of moving as a unit, and the vibra- 
tion frequencies of such segments will be 
higher than that of the drum membrane as a 
whole. Similarly, the tectorial membrane, even 
though it is relatively flaccid and does not 
hinder transmission of pressure changes to the 
basilar membrane, tends to load the latter dif- 
ferently during positive and negative half 
cycles of the incident disturbance. 



Chapter 2 

BEHAVIOR OF THE EAR 


T he fundamental behavior of the ear de- 
pends on the structural factors discussed in 
the preceding chapter and sheds further light 
on the problems already discussed. The present 
analysis is more immediately concerned with 
what rather than how we hear and gathers to- 
gether data which will be useful in connection 
with problems studied later in this volume. The 
chief points around which the present chapter 
is developed are: the faintest wanted sounds 
audible in the presence and absence of inter- 
fering, or unwanted, sounds ; sensations of 
pitch and loudness; the auditory effects of 
sounds containing many component frequen- 
cies; and the sensed effects produced by vary- 
ing the durations of the sounds and their com- 
ponents. 

2 1 THRESHOLD FOR TONES 

Four major types of threshold may be dis- 
tinguished. Of these, three kinds of absolute 
threshold are described in Section 2.1.1. (An 
absolute threshold is the critical level of a 
sound when no background is present.) A 
fourth variety, the threshold in the presence of 
masking tones, is defined and discussed in Sec- 
tion 2.1.2. Thresholds for pure tones are con- 
sidered first in the present section, whereas 
complex sounds are considered in Section 2.3. 

^ ^ ^ Absolute Threshold 

The first of the three absolute thresholds is 
called the audibility threshold, or the absolute 
audibility threshold, and refers to the smallest 
rms pressure which a pure tone must have in 
order to be heard by an average, normal ear, 
provided the surroundings are quiet, so that the 
listener experiences no interference from un- 
wanted sounds. The second kind of threshold 
refers to excitation of the hearing mechanism 
by methods other than varying the air pressure 
in the ear canal. The third kind of threshold, 
variously termed the threshold of pain or the 


12 


threshold of feeling, refers to sensations of a 
nonauditory character which are produced by 
very intense airborne sounds. 

It should be understood that all these thresh- 
olds actually refer to the midregions of zones 
within which certain phenomena become more 
or less well defined and do not specify anything 
more than average values. Consequently it is 
important to state in all cases the conditions of 
measurement and the variability of observer 
response. 

Audibility Threshold 

Figure 1 shows the audibility threshold at 
various frequencies for three experimental 
situations. These thresholds indicate the re- 
sponse of acute ears when the static pressure is 
about 760 millimeters. Since these results were 
obtained from a large number of observers, 
individual peculiarities, which are character- 
istic of outer ear resonances, have been aver- 
aged out. It is interesting to note that the 
threshold of hearing is much the same at static 
pressures corresponding to an altitude of 
36,000 feet above sea level.^ 

The general shape of the threshold curve im- 
plies that the overall response of the ear mecha- 
nism as a whole, consisting of the outer, middle, 
and inner portions coupled together, resembles 
that of a broadly tuned resonator whose peak 
response lies in the interval between 1 and 7 
kilocycles. That this response characteristic is 
not associated exclusively with only one por- 
tion of the ear is indicated by the effect of sur- 
gical changes in the shape of the outer ear 
canal (see Section 1.5.1) and also by the fol- 
lowing set of observations made in cases in 
which the experimenter had direct access to the 
middle ear cavity.- Increasing the effective 
stiffness of the chain of ossicles by inserting a 
tuft of compressed cotton between them in- 
creased the acuity of the ear to high frequen- 
cies. This would be expected from the fact that 
the natural frequency of a mechanical reso- 




RESTRICTED 


THRESHOLD FOR TONES 


13 


nator increases when the stiffness, or coefficient 
of restoring force, is increased (see equation 
(1) in Chapter 1, for example). In addition, 
when the subjects reclined on their sides and 
a drop of mercury was placed on the round 


sponse is mass controlled at high frequencies 
(response falls for high accelerations due to 
inertia) and stiffness controlled at low fre- 
quencies (when inertia is unimportant, the role 
of stiffness is more prominent). For frequen- 



1 FREE FIELD THRESHOLD - FACING SOURCE 

2 FREE FIELD THRESHOLD - RANDOM INCIDENCE 

3 HEADPHONE THRESHOLD 

4 RAIN THRESHOLD 


Figure 1. Audibility thresholds at different frequencies. (Courtesy John Wiley and Sons.) 


window, the increase of mass increased acuity 
for low frequencies. Similar results have been 
obtained in animal experiments in which tissue 
was grafted over the round window and the 
cochlear potentials measured before and after 
the graft was made.^ 

The results of impedance measurements 
made upon the ear^ also indicate that its re- 


cies at which the effects of stiffness and mass 
are balanced, or nearly so, the system is in 
resonance. 

Curve 1 in Figure 1 was obtained by deter- 
mining the pressure at a stated position with 
respect to a source of single-frequency sound. 
After the level of the sound in this free field 
had been measured, the heads of the observers 


14 


BEHAVIOR OF THE EAR 


were placed at the reference position, facing 
the source, and their ability to hear the gen- 
erated tones was noted. This curve, as well as 
curves 2 and 3 in the same figure, shows that 
the faintest audible tones are those with fre- 
quencies in the neighborhood of 3 kilocycles. 
At this frequency, the faintest audible sounds 
have a free-held pressure of about 80 decibels 
below 1 dyne per square centimeter, as shown 
by the right-hand scale. This corresponds to an 
rms pressure Po of 10-^ dyne per square centi- 
meter, in other words, 20 log (1/10-^) = — 80 
decibels. Since the intensity is equal to Po^/pC, 
where p, the density of air at 760 millimeters 
and 20 C, is 1.2 XlO-^ gram per cubic centime- 
ter, and c, the velocity of sound propagation 
under the same conditions, is 3.4X10^ centime- 
ters per second, it follows that the power sup- 
plied to the minimum audible field amounts to 
(10-^)V[(1.2 X 10-^) (3.4X10^)], or 2.5 X 

10-10 QYg pgy second per square centimeter. This 
is equal to 2.5 X 10“^^ watt per square centi- 
meter. 

Since the intensity (2.5 X 10“^° erg per second 
per square centimeter) may also be expressed 
as the quantity ^ir^f-A^pC, where /, the fre- 
quency of the tone, is 3 kilocycles, A is the peak 
displacement amplitude, and the other symbols 
have the significance already stated, the maxi- 
mum displacement of an air particle in the 
threshold sound field may be readily computed. 
Thus 

2 5 X 10“^° = 

27r2(3 X 103)2^2(1.2 X lO-^) (3.4 X 10^) 
or 

A = 2 X 10“io cm. 

Attempts to measure the displacement of the 
eardrum at threshold have given roughly this 
same value at frequencies between 1 and 10 
kilocycles. Thus the maximum displacement at 
threshold is at most of molecular dimensions. 

It may also be shown^ that the intensity of 
the noise generated by the random thermal 
motion of the air particles is of the order of the 
intensity of the faintest audible 3-kilocycle 
tone. Thus, a further increase in the acuity of 
the ear in the region between 1 and 5 kilocycles 
would not make it possible to hear fainter 
wanted sounds, since these would be drowned 
out by the thermal noise in the air and in the 
structures of the ear. 


The irregular shape of curve 1 in the regions 
centered at 1 and 6 kilocycles is due to the fact 
that the intrusion of the observer changes the 
distribution of energy in the sound field. These 
irregularities in curve 1 are associated with the 
effects of diffraction around the head and 
shoulders of the observer. The diffraction pat- 
tern changes with the frequency of the source 
and the orientation of the head relative to the 
source. Thus, when the sound is propagated 
toward the side of the head (that is, directed 
upon the ear) instead of toward the face, the 
minimum audible field for a 3-kilocycle tone is 
nearly 100 decibels below 1 dyne per square 
centimeter and is proportionately lower than 
indicated by curve 1 for frequencies above and 
below 3 kilocycles. 

Curve 2 shows the effect of correcting for 
diffraction and has been computed by assum- 
ing that the head is immersed in a random, or 
nondirectional, sound field. Since the sound 
shadow cast by the head is negligible for low 
frequencies, curves 1 and 2 do not diverge ap- 
preciably before the source frequency exceeds 
500 cycles per second. In general, curve 2 is 
lower than curve 1 because more sound reaches 
the ear at an optimal orientation in the case of 
the random sound field. 

Curve 3 indicates the absolute threshold de- 
termined with a headphone applied to one ear. 
This monaural threshold was obtained by de- 
termining the pressure in the enclosure formed 
by the tightly applied headphone. Such pres- 
sure measurements are very difficult to make at 
the low values required. The threshold pressure 
was actually found by a theoretical computa- 
tion, based on knowledge of the volume of en- 
closed air and the displacements of the head- 
phone diaphragm. This method involves two 
assumptions. In the first place, leakage of air 
along the headphone seal is neglected, which is 
probably not justified at the lowest frequencies. 
In the second place, the method assumes that 
the sound pressure is a linear function of the 
electrical current fed into the headphone, since 
the displacement of the headphone diaphragm 
is assumed proportional to the current; this 
assumption may not be legitimate for the entire 
range of sound pressures used. 

The shape of curve 3 is in general the same 
as that of curve 2, but the monaural pressure 


RESTRICTED 


THRESHOLD FOR TONES 


15 


measurements are, on the average, about 15 
decibels above the binaural field measurements. 
There are many factors which tend to produce 
this shift, but it is not known whether they 
account for all the difference between curves 2 
and 3. Thus, scattering of sound into the ear 
canal by the outer ear structures (see Section 
1.5.1) increases with increasing frequency, and 
resonances in the outer ear also tend to amplify 
the pressure at the eardrum with respect to 
the pressure measured in the free field. 
Furthermore, unless echoes from the walls of 
the test enclosure are completely eliminated, 
they may return toward the sides of the head 
in a manner different from that by which they 
reach the pickup used in the free-held calibra- 
tion. Such echo effects have been found capable 
of depressing the free-held threshold by 3 to 
5 decibels. 

Differences between curves 2 and 3 at fre- 
quencies below 1 kilocycle are not likely to be 
due to the factors mentioned above. On the 
other hand, there are at least three effects 
known to occur at these low frequencies which 
would be expected to produce differences be- 
tween the headphone and free-held thresholds; 
namely, rehex tensing of the middle ear 
muscles, physiological noise, and acoustic leak- 
age. Rehex tensing of the middle ear muscles 
occurs in the case of loud sounds, and this ten- 
sion increases the acoustic impedance of the 
ear for frequencies below 1 kilocycle. Since 
the headphone pressure measurements are made 
by determining the electrical energy fed to the 
receiver at threshold and also about 60 decibels 
above threshold, the possibility that the middle 
ear muscles are in different states of tension at 
these two levels means that the acoustic power 
used to move the eardrum is not a linear func- 
tion of the electrical power fed to the head- 
phone. 

The term “physiological noise” refers to me- 
chanical vibrations set up in the space enclosed 
by the tightly fitting headphone due to pulse 
actions, motion of air through the head cavities 
during Jthe act of breathing, and similar phe- 
nomena. Such noise would tend to drown out 
the wanted sound and elevate the low-fre- 
quency threshold. 

Even when the headphone cap fits fairly 


tightly, leakage of air along the seal is likely 
to be greater during the long period of a low- 
frequency tone than during the short period of 
a high-frequency sound. This leakage will tend 
to make the actual pressure change in the ear 
canal smaller than the computed changes. 

Two additional aspects of these threshold 
measurements should be mentioned at this 
point: (1) the difference between monaural 
and binaural thresholds and (2) the element 
of observer variability. It has been shown® that 
the total energy required to reach threshold by 
means of a tone led to one ear is equal to the 
sum of the energies of tones led independently 
to both ears. Furthermore, this equality is in- 
dependent of the numerical ratio in which the 
energy is divided between the two ears. In fact, 
the frequencies as well as the intensities of the 
tones led to the two ears may differ from each 
other over a fairly wide range and the binaural 
threshold will still be reached at the same 
value of the total intensity, provided the two 
ears are fairly well matched with respect to 
acuity. These observations imply that the asso- 
ciative centers in the brain integrate the nerve 
discharges produced by stimulation of both 
ears, and there is other evidence supporting 
this view (see Section 8.1.3). In addition, the 
results just quoted indicate that the binaural 
threshold should lie some 2 to 3 decibels below 
the monaural threshold. 

As to observer variability, it should be noted 
that the position of the threshold varies from 
day to day for a given observer, and even from 
minute to minute. Thus, if a sustained tone is 
presented to an observer at a level near thresh- 
old and he is told to press a key whenever the 
tone is audible and to release it when the tone 
is inaudible, he will in general press the key 
intermittently. Similarly, when groups of tonal 
pulses with intensities near threshold are 
presented and the observer compares the dura- 
tion and succession of his acoustic sensations 
with the actual duration and grouping of the 
tonal stimuli, it is found that he is, in general, 
unable to hear all the pulses in a group, and 
that the sensed duration of audible pulses is 
less than their actual duration.'^ The same effect 
was observed when the threshold had been 
artificially elevated by strong stimulation. It 


lESTRICTED 


16 


BEHAVIOR OF THE EAR 


follows that the level of threshold must be 
defined in statistical terms. Unless experimental 
measurements of threshold refer to the same 
probability of perceiving the test sound, such 
measurements are not strictly comparable, and 
in extreme cases may vary from each other by 
as much as 10 to 15 decibels. 

Because of the spread in the measurements, 
an average between curves 2 and 3 is generally 
adopted, and the threshold for a 1-kilocycle 
tone is usually defined as 74 decibels below 1 
dyne per square centimeter, in other words, 
2X10'^ dyne per square centimeter. In many 
types of work this rms pressure value is used 
as the reference standard, and therefore cor- 
responds to a level of 0 decibels (see left-hand 
ordinate in Figure 1). The convenience of this 
standard is largely associated with the fact 
that most sound levels of interest will have 
positive values. 

The decibel scale expresses the value of a 
ratio. In underwater acoustics, the conven- 
tional standard which is used to form this ratio 
is an rms pressure of 1 dyne per square centi- 
meter ; in atmospheric acoustics, it is customary 
to use 2 X 10^^ dyne per square centimeter as 
the reference standard. Since the present dis- 
cussion is concerned with hearing as well as 
with underwater sounds, both scales are given 
wherever they are relevant. The air standard 
is used in all scales on the left of the diagrams ; 
the water standard is printed at the right. Any 
given pressure p is 74 decibels higher on the 
“air’’ scale than it is on the “water” scale. 

Bone Conduction Threshold 

In some diseases of the ear, surgical removal 
of the ossicles (except for the fragment which 
seals the oval window) is necessary. While loss 
of the ossicles raises the audibility threshold 
by about 60 decibels, it does not produce com- 
plete deafness. The available evidence indicates 
that the inner ear mechanism is stimulated in 
such cases by vibrations of the skull structure 
in response to the incident airborne sound. 
Thus, as long as the inner ear and the auditory 
nerve remain intact, and the osseous pathway 
has not been impaired by skull fracture, it is 
always possible to deliver sound vibrations at 
sufficient intensity to permit useful hearing. 


Bone conduction thresholds are usually deter- 
mined by applying the end of a vibrating rod 
to the chin, the forehead, or the mastoid proc- 
ess. For individuals with normal hearing, the 
bone conduction threshold, plotted as displace- 
ment amplitude versus frequency, continues to 
be concave upward as in Figure 1; but the 
curve tends to be nearly flat between 1 and 6 
kilocycles,® owing to the fact that there is vir- 
tually no resonance amplification such as is 
normally introduced by the outer ear. At 
threshold, for a frequency of 800 cycles per 
second, the amplitude of motion of the forehead 
immediately adjacent to the vibrating rod has 
been found to be 3.5 X 10-^ centimeter in the 
case of normal ears® and is probably nearly the 
same when the ossicles have been removed. The 
necessary displacement amplitude falls to 
5 X 10-^® centimeter, if the ear canal is closed by 
inserting the finger into it. Under these circum- 
stances, the sealed-off air is set into vibration,^® 
the eardrum moves, and energy is transmitted 
to the cochlea through the ossicles. 

Motion of the basilar membrane occurs in 
the case of bone-conducted vibration because of 
two asymmetries in the structure of the cochlea. 
Thus, compression of the semicircular canals 
increases pressure to a greater extent in the 
upper gallery of the cochlea than in the lower 
(see Figures 2 and 6 in Chapter 1). Similarly, 
the round window yields more readily than the 
oval window when the cochlea is compressed, 
and this also tends to establish a pressure 
gradient across the basilar membrane, directed 
from the upper to the lower gallery. 

The fact that bone conduction occurs means 
that airborne sounds produced in noisy loca- 
tions will interfere with the audibility of sig- 
nals brought to the ear by means of head- 
phones. Under ideal conditions, therefore, the 
insulation afforded by headphones against un- 
wanted airborne sounds is not likely to exceed 
60 decibels and will usually be less. 

Since air and bone conduction of a given tone 
stimulate the same portion of the basilar mem- 
brane, it is possible, by adjusting the* phases 
and intensities of simultaneous osseous and 
atmospheric stimuli, to make the subjective 
sensation disappear.^^ 


THRESHOLD FOR TONES 


17 


Pain Threshold 

Tests conducted with airborne sounds of fre- 
quencies between 1 cycle and 8 kilocycles show 
that nonauditory sensations — tactile sensa- 
tions, feelings of pressure, thrusting, tickling, 
itching, and burning — appear whenever the 
intensity of the stimulus tone reaches a value 
of about 130 decibels above 2 X 10-^ dyne per 
square centimeter, as indicated by curve 4 in 
Figure 1. Since permanent injury results from 
continued exposure to sounds of higher inten- 
sity, the useful range of auditory stimuli lies in 
the area defined by the threshold of hearing and 
the threshold of pain. 

^ ^ ^ Masked Threshold 

It is a common experience that the level of 
the voice must be raised if the speaker wishes 
to be heard at a noisy location. This phenome- 
non is termed masking. Quantitative studies of 
masking have provided much useful informa- 
tion concerning the behavior of the ear; hence 
the masking effect of one tone upon another 
will be described in the present section. 

The unwanted sound which produces the 
masking will be called the background (or the 
masking tone) and the wanted sound will be 
called the signal (or masked tone). Signal or 
background, or both, may be atonal sounds, and 
the masking produced by and upon such sounds 
is discussed in subsequent sections. 

In general, the background must be audible, 
otherwise it will not produce masking. Since 
the effect of a masking tone is usually to raise 
the audibility threshold of the signal, it is 
customary to define the amount of masking by 
specifying the difference in decibels between 
the level of the signal when it is (1) presented 
to the ear at a just audible level without inter- 
ference from masking, that is, the level at the 
absolute audibility threshold, and (2) presented 
to the ear at sufficient strength to become just 
audible in the presence of the masking tone, 
that is, the level of the masked threshold. 

The level of the audibility threshold depends 
only on the acuity of the ear; the level of the 
masked threshold depends also upon the prop- 
erties of the background. In effect, the masking 


sound produces a temporary deafness to the 
signal by shifting its audibility threshold. 

A group of charts indicating the threshold 
shifts produced by masking are shown in Fig- 
ure 2. The frequency of the masking tone is 
indicated at the top of each chart, and a num- 
ber specifying the sensation level, or relative 
intensity, of the masking tone is used to label 
each masking curve. By sensation level is meant 
the number of decibels by which the level of the 
masking tone exceeds its own threshold level. 
From Figure 1, it will be seen that when a 200- 
cycle tone has a sensation level of 80 decibels. 
It is about 110 decibels above 2 X 10^^ dyne per 
square centimeter whereas a 1,000-cycle tone 
at a sensation level of 80 decibels is only 80 
decibels above 2 X 10 ^ dyne per square centi- 
meter. Thus, the sensation level of a tone does 
not directly state either its objective intensity 
or, as will be shown later, its subjective loud- 
ness ; nonetheless, it is a precise and useful con- 
cept. 

The frequency of the masked tone is shown 
on the horizontal scales in Figure 2, and the 
threshold shift of the masked tone (the sen- 
sation level of the signal just audible in the 
presence of the indicated background) is given 
by the vertical scales. The threshold shifts were 
obtained by testing monaurally the ears of 
groups of normal observers. 

The signal and background tones were gen- 
erated by electrical oscillators, freed of their 
harmonic content by filtering, and fed to head- 
phones. The power input required to reach the 
audibility threshold was determined individu- 
ally for each of the signal and background 
tones. In addition, a mixture of a background 
tone (at a fixed intensity) and one signal 
(whose intensity could be varied gradually) 
was applied to the headphones. The threshold 
shift was then obtained by noting the power 
supplied by the signal oscillator when the 
masked signal could be heard half the time by 
the various observers, or by the same observer 
in half the tests. 

This averaging procedure is necessitated by 
the fact that the masked threshold is not infi- 
nitely sharp. As the signal-to-background ratio 
is gradually increased the signal undergoes a 
transition (extending over a range of about 12 


^ll^TRICTED 


18 


BEHAVIOR OF THE EAR 



FREQUENCY IN CYCLES 



O 


8 ^ S 

FREQUENCY IN CYCLES 


O 

O 

u> 

lO 


o 

O 

O 



FREQUENCY IN CYCLES 



° 8 


o 

o 

00 


FREQUENCY IN CYCLES 


O 

O 




O 

o 

o 

•t 



Figure 2. Masking of tones by tones (200, 400, 800, 1,200, 2,400, and 3,500 cycles). (Courtesy D. 
Van Nostrand Co.) 


v 


RESTRICTEIT 




THRESHOLD FOR TONES 


19 


decibels; see Section 4.1.4, for example) from 
“inaudible,'' through “audible some fraction of 
the time," to “audible all the time." It is there- 
fore convenient to adopt the signal-to-back- 
ground ratio at which the signal is audible in 
one-half, or some other stated fraction of the 
trials as defining the masked threshold. When 
defined in this manner, the level of the masked 
threshold for a specific signal-background com- 
bination can usually be duplicated to within 1 
or 2 decibels in successive determinations. 

Three general conclusions may be drawn 
from these charts. First, the threshold shift is 
greatest at frequencies adjacent to that of the 
background. Secondly, the threshold shift at 
remote frequencies increases when the sensa- 
tion level of the background is raised, especially 
when it is larger than 40 decibels. Finally, the 
threshold shifts at remote frequencies are more 
severe in the direction of high frequencies. 

It should be stated at this point that the 
masking curves obtained are significantly dif- 
ferent from those shown in Figure 2 when the 
masking tone is introduced into one ear and 
the masked tone into the other. In general the 
level of the masking tone presented to one ear 
must be about 50 decibels higher than that 
shown in Figure 2 in order for it to produce the 
indicated threshold shifts of the masked tone 
led to the other ear. The available evidence in- 
dicates that this binaural masking effect does 
not occur in the brain, but rather that bone con- 
duction occurs, and the masking produced in 
the ear to which the signal is presented arises 
from the bone-conducted sound transmitted 
across the head from the opposite ear. The at- 
tenuation factor of 50 decibels agrees with the 
bone-conduction data described in Section 2.1.1. 
This conclusion agrees with observations made 
on individuals with one normal ear and one 
totally deaf ear. When a telephone receiver is 
held to the deaf ear in such cases, the level of 
the presented tone must be raised approxi- 
mately 50 decibels above normal threshold in 
order to be heard. That this tone is actually 
heard in the normal ear is indicated by the 
fact that its loudness is increased by inserting 
a finger in the canal of the normal ear. 

It should also be added that the masking data 
shown in Figure 2 apply only when signal and 


background are presented simultaneously and 
have a duration of between 1 and 10 seconds. 
For presentation intervals of different length 
and for successive rather than simultaneous 
presentation of the two sounds, other factors 
strongly influence the results. 

For example, it has been observed^^ ^^^t the 
threshold shift of a 2,250-cycle signal, masked 
by a 1-kilocycle background is not independent 
of time. In these experiments, the 1-kilocycle 
background was presented continuously and the 
masked threshold of the signal was determined 
at some known time after the background was 
turned on. When the signal threshold was de- 
termined 10 seconds after the onset of back- 
ground, the shift amounted to about 12 decibels. 
The threshold shift grew progressively smaller 
with time and remained substantially constant 
— at a value of 4 decibels — for all deter- 
minations made more than two minutes after 
the background was first turned on. During the 
entire test period the intensity of the 1-kilo- 
cycle background was maintained constant, at a 
sensation level of about 50 decibels. 

That this diminution of threshold shift was 
not due to practice in making successive deter- 
minations is indicated by the fact that the same 
low value of 4 decibels was found even when 
the first test was made after the observer had 
listened passively to the uninterrupted back- 
ground for about five minutes. Since the sensed 
character of the sustained background changed 
with time — it is described as growing “more 
mellow" and “less insistent" — the experi- 
menters concluded that the reduction of mask- 
ing was due to fatigue. According to this view, 
the acoustic nerve became less responsive to the 
sustained background and hence relatively 
more responsive to the signal. 

It seems to follow that (1) the signal-to- 
background ratio corresponding to the masked 
threshold would be independent of time if the 
signal were also a sustained sound, and (2) the 
characteristics of masking are determined by 
the nature and behavior of the auditory nerve 
tract as well as by the dynamics of the cochlea. 

Adjacent and Remote Masking 

If the basilar membrane had perfect se- 
lectivity — if, in other words, the motion of a 


iA^TRICT^ 


20 


BEHAVIOR OF THE EAR 


particular auditory string could be excited only 
when a tone of its own frequency reached the 
ear — the masking effect of any background 
tone would be confined to signals of identical 
frequency. As Figure 2 shows, the tuning is 
not perfect. Most of the masking does seem to 
be confined to signals with frequencies imme- 
diately adjacent to that of the background, 
especially for backgrounds of low intensity, and 
this explains why it is often helpful to change 
the pitch of the voice when speaking in noisy 
surroundings. This type of masking is called 
adjacent masking. However, as the sensation 
level of the background increases, its masking 
effect extends to signals with frequencies more 
and more remote from that of the masking 
tone. In other words, when a tone has a suffi- 
ciently high intensity it produces motion of a 
large fraction of the basilar membrane. This 
masking of distant frequencies is called remote 
masking. These two concepts, remote and adja- 
cent masking, are helpful in discussions of 
masking data. 

Beats 

The reason that the masking curves show a 
dip, or relative minimum, instead of having a 
single peak at one frequency of the background, 
where this sound produces its maximum stimu- 
lation, is that beats are heard when the signal 
and background tones differ in frequency by a 
small number of cycles per second. The audi- 
bility of the beats makes it much easier to 
detect the presence of the signal, since they 
produce easily recognized changes of loudness. 
When the signal is very brief, the beats cannot 
be heard, and the masking curve has a single 
maximum at the frequency of the background 
(see Figure 5 in Chapter 10). 

The occurrence of audible beats when two 
tones of comparable intensity are sounded to- 
gether indicates that each tone stimulates a 
finite segment of the basilar membrane. When 
these segments of the membrane overlap, the 
net disturbance due to the simultaneous action 
of both tones will vary with time, being greatest 
when the two stimulated patches move in phase 
and least when they are of opposite phase. 
Thus, the net disturbance of the membrane de- 


pends significantly on the relative phases of the 
two tones, but only when these tones are suffi- 
ciently close in frequency so that they can 
stimulate the same portion of the basilar mem- 
brane. In other words, the occurrence of beats 
indicates that the ear is not a perfect analyzer ; 
that is, it does not have infinitely sharp tuning. 

This fact also explains the observation that 
the threshold shift is sometimes negative, that 
is, the signal may become audible below its ab- 
solute threshold when the presence of a back- 
ground tone, sufficiently close to it in frequency, 
causes the total stimulation of the patch As to 
reach threshold. 

According to this view, the masking curves 
would bear a close resemblance to simple reso- 
nance curves, at least for background levels 
below 50 decibels, if the loudness changes due 
to beats did not produce a dip at frequen- 
cies adjacent to that of the masking tone. The 
reason for changes in the appearance of the 
masking curves when the sensation level of 
background exceeds 50 decibels is the fact that 
the nonlinearity of the ear introduces har- 
monics of the masking tone, as discussed on 
page 24 ff. 

Thus, Figure 3 has been redrawn from Fig- 
ure 2 in order to show the type of curve which 
would be obtained if no assistance were af- 
forded by beats. The peak level of the hatched 
portion of each of these curves at the frequency 
of background has been set equal to the sensa- 
tion level of the background. This procedure 
assumes that ability to hear the signal (rather 
than beats associated with its presence) is pos- 
sible if the stimulation of the basilar membrane 
produced by the signal is equal, for at least one 
patch of the membrane, to the stimulation 
produced there by the background. The as- 
sumed identity of masking and stimulation has 
proved to be a useful principle in the solution 
of a variety of problems. The peak levels of 
darkened areas corresponding to harmonics of 
the masking tone have been derived from mea- 
surements described on page 24 ff. 

It will be seen that the height and base width 
of each of the hatched areas increases with the 
intensity of the masking tone. Both of these 
circumstances are associated with the audibility 
of beats. Consider, for example, the height of 


THRESHOLD FOR TONES 


21 


the hatched area (24 decibels from the bottom 
of the dip to the top of the peak) for the case 
of an 800-cycle tone at a sensation level of 60 
decibels. If the enhanced audibility of signals 


smallest change of intensity which the ear can 
detect amounts to about 25 per cent. In other 
words, when the intensity of an 800-cycle tone 
at a level 60 decibels above threshold is period- 



FREQUENCY IN CYCLES 

Figure 3A. Assumed stimulation of the basilar membrane at different sensation levels for an 800- 
cycle tone. 



0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 

FREQUENCY IN CYCLES 

Figure 3B. Assumed stimulation of the basilar membrane at different sensation levels for a 2,400- 
cycle tone. 


which differ from background by 2 to 3 cycles 
per second is due to the perception of loud- 
ness changes, the improvement of 24 decibels 
should be determined by the fact that under 
these circumstances (see Section 2.2.2) the 


ically varied three times per second so that its 
highest intensity is 1.25 times its lowest inten- 
sity, the fluctuation of level is just detectable. 

This fluctuation may also be considered to 
arise from the mixture of an 800-cycle back- 



DISPLACEMENT OF MEMBRANE, EXPRESSED DISPLACEMENT OF MEMBRANE, EXPRESSED 

AS FRACTION OF MAXIMUM DISPLACEMENT AS FRACTION OF MAXIMUM DISPLACEMENT 


22 


BEHAVIOR OF THE EAR 



A 



0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 

DISTANCE FROM APEX OF BASILAR MEMBRANE IN MM 

B 


Figure 4, Displacement patterns produced by an 800- and a 2,400-cycle tone, each at a sensation 
level of 80 decibels. These patterns have been computed from Figure 3. 



THRESHOLD FOR TONES 


23 


ground with a 797-cycle or an 803-cycle signal. 
Let the peak pressure of the 800-cycle back- 
ground be P and the peak pressure of the sig- 
nal be p. Then the peak intensity of the mix- 
ture will be proportional to (P + p)^, and the 
minimum intensity to (P — p)^. When the ratio 


the periodic fluctuation of intensity will be just 
audible. Therefore, p/P = 0.052, and 20 log 
0.052 = —25.6 decibels, which is in good agree- 
ment with the observed fact that the level of 
the just audible signal is 24 decibels below the 
level of background when the two tones have 
nearly identical frequencies. 

The increase in the height of the hatched 
area with sensation level agrees with the ob- 
servation (see Section 2.2.2) that the smallest 
audible change of intensity is a smaller fraction 
of the comparison intensity when the latter is 
raised. Similarly, the decrease in the height of 
the hatched area with increasing difference be- 
tween the frequencies of signal and background 
(and hence, an increase in the rate of beating) 
is due to the fact that it is more difficult to de- 
tect intensity fluctuations when the rate of 
fluctuation increases (see Figure 16). In addi- 
tion, the edge of a hatched area marks the point 
where the rate of beating is too great and the 
overlap of the patches of membrane stimulated 
by signal and background too small to permit 
detection of the signal by means of periodic 
changes of loudness. 

It should be noted that the curves in Figure 2 
are plotted to a logarithmic ordinate in order to 
reduce them to a manageable scale. Further- 
more, the abscissa gives the frequency, rather 
than the position along the basilar membrane. 
If it is assumed that the maximum displace- 
ment produced by the background at any point 
on the membrane is proportional to the thresh- 
old shift (expressed in terms of pressure) and 
that the position on the basilar membrane cor- 
responding to any frequency is given by 
equation (5) of Chapter 1, Figure 4 may be 
drawn. This figure shows the estimated dis- 
turbance patterns produced by an 800- and a 
2,400-cycle tone, each at a sensation level of 80 
decibels, at the moment when the centers of the 
vibrating patches reach their maximum dis- 


placements simultaneously. Since the data on 
which the 2,400-cycle figure is based do not ex- 
tend beyond 4 kilocycles, no harmonics are 
shown in this case. 

The width of a resonance peak is usually 
measured between points at which the response 
is half the maximum value. Using this crite- 
rion, it will be seen that the width of a peak at 
any frequency is about 6 X 10-^ centimeter. The 
width of the region stimulated by a harmonic 
at any frequency has about this same value. 
It may be noted that this value is probably an 
overestimate, since the widths of the peaks 
shown here are based on the presence of beats 
and thus involve the overlapping of two stimu- 
lated patches of membrane. Hence, the stimu- 
lation pattern produced by a single tone is 
probably more nearly 3 X 10-^ centimeter in 
width. As shown by Figure 3, the widths of the 
peaks increase somewhat with intensity. The 
indications that patch width is relatively inde- 
pendent of stimulus frequency and that it has a 
numerical value of about 3 X 10“^ centimeter, or 
1 per cent of the total length of the basilar mem- 
brane, are in good agreement with the evidence 
discussed in Sections 2.2.1 and 2.3. 

By way of conclusion, it is worth describing 
the sensations produced by two tones of fre- 
quency /i and /s that produce beats. When the 
frequency difference between the tones is less 
than 1 cycle per second, the sensation is that 
of a succession of smooth increases and de- 
creases in loudness. This loudness variation is 
most noticeable when the rate of beating is be- 
tween 2 and 3 cycles per second (see also Sec- 
tion 2.2.2) and the intensities of the stimulus 
tones are equal. At rates of between 2 and 7 
beats per second the sensed pitch of the beating 
complex seems to lie midway between the two 
stimulus tones and is therefore called the inter- 
tone, i.e., the maximum stimulation of the mem- 
brane occurs at a point between the centers of the 
patches disturbed by each of the beating tones. 

At about 7 beats per second, the periodic 
variation in loudness ceases to have a smooth 
growth and decline, the beats are heard as a 
series of intermittent impulses, and the pitches 
of the two stimulus tones are heard in addition 
to that of the intertone. As the rate is increased 
to about 15 per second, the intermittence asso- 
ciated with the individual beats begins to sound 


24 


BEHAVIOR OF THE EAR 


like a flutter. For rates exceeding 20 per second, 
the intertone ceases to be heard, leaving only 
the two stimulus tones, and the sensation is as- 
sociated with a “roughness” or a “rattle” which 
makes such combinations extremely dissonant, 
especially in the region of 30 to 70 beats per 
second. 

For sufflciently large frequency separations, 
the roughness ceases to be heard. The cessation 
of roughness is associated with the fact that 
the disturbed patches of the membrane cease to 
overlap to any signiflcant extent. These patches 
have approximately the same widths for all 
stimulus frequencies, but the frequency inter- 
val contained in each patch increases with the 
frequency of the stimulus tone (see equation 
(5) of Chapter 1 or Figure 9). Hence, the fre- 
quency difference (fo — A) between the stimu- 
lus tones for which roughness vanishes in- 
creases as the frequencies A and A in- 
creased, as shown by Table 1. 


Table 1. Roughness due to beats. 


Frequency of 

Number of beats per second 

lower tone 

Greatest 

Vanishing 

in cycles 

roughness 

roughness 

96 

16 

41 

256 

23 

58 

575 

43 

107 

1707 

84 

210 

2808 

106 

265 


The sensitivity of the ear to beats has re- 
ceived an interesting application in the detec- 
tion of methane (or Are damp) in coal mines. 
Since methane is less dense than air, sound is 
transmitted through it with greater velocity; 
hence, an organ pipe filled with methane, or an 
air-methane mixture, has a higher pitch than a 
similar pipe filled with pure air. When two such 
pipes, one of them filled with the suspected 
mine air and the other with pure air, are 
sounded simultaneously, beats will be heard 
when significant quantities of methane are 
present. The concentration of the methane can 
be estimated from the rate of beating. 

Beats may of course occur directly between 
the stimulus tones, or between such a tone and 
an aural harmonic of the second, or between 
the aural harmonics of both. The nature of 
aural harmonics is discussed in the following 
section. 


Aural Harmonics 

When a pure tone of high intensity stimu- 
lates the ear, the sensed sound contains fre- 
quencies which are harmonics of the stimulus 
tone and which are not present in the objective 
stimulus. Thus, when an intense 200-cycle tone 
is presented, the observer can also hear tones 
of 400 and 600 cycles per second. 

The aural harmonics of the stimulus have 
been investigated by means of probe tones, 
which give audible beats when their own fre- 
quencies are adjacent to those of the aural har- 
monics. In fact, this is essentially the meaning 
of the dips in the masking curves at multiples 
of the background frequency. 

The absence of dips at multiples of the mask- 
ing frequency for the 200-cycle and 400-cycle 
background tones in Figure 2 should not be 
construed to mean that aural harmonics do 
not occur for low-frequency tones. Actually, 
aural harmonics are quite prominent for the 
low-frequency sounds and do not appear in the 
two charts mentioned because the curves were 
smoothed for purposes of reproduction. 

Figure 5 shows the harmonics of back- 
grounds of 75, 250, and 500 cycles in some de- 
tail, together with the masking produced by 
these tones.^^ The dashed segments of the mask- 
ing curves define the regions in which the sig- 
nal was detected by means of beats. The height 
of the vertical line at the fundamental or mask- 
ing tone frequency indicates the sensation level 
of the background. The heights of the vertical 
lines at the harmonic frequencies indicate the 
sensation levels of the signals which gave the 
strongest beats. Since the strongest beats occur 
when objectively presented beating tones have 
equal amplitudes, the heights of the vertical 
lines are assumed to indicate the sensation level 
of the aural harmonic. 

This line of reasoning is indirect, but its re- 
sults are in agreement with direct measure- 
ments of harmonics in the cochlear potentials, 
as described below. The agreement between 
these two sources of evidence indicates the 
validity of the best beat method, which equates 
stimulation and masking, and thereby supports 
the assumption used in constructing Figures 3 
and 4. 


RESTRIGT^t r 


THRESHOLD FOR TONES 


25 




0 200 400 600 800 1000 1200 1400 


FREQUENCY IN CYCLES 



FREQUENCY IN CYCLES 


Figure 5. Masking of tones by tones (75, 250, and 500 cycles). (Courtesy Journal of the Acousti- 
cal Society of Ameynca.) 


RESTRIGTED 


26 


BEHAVIOR OF THE EAR 


The results of analysis of aural harmonics by 
means of the method of beats are summarized 
in Figures 6, 7, and 8. These figures are based 
on a study of tones with frequencies between 
50 cycles and 8 kilocycles, and with intensities 


monies is given by the vertical scale. It will be 
seen that, for all the frequencies studied, the 
magnitudes of successive harmonics depend 
only on the intensity of the fundamental and 
not on its sensation level or frequency. Since 



10 1 s V ■'■■J J J 

01 234567 89 10 

NUMBER OF HARMONIC 


Figure 6. Relative intensities of aural harmonics produced by fundamentals of different intensi- 
ties. (Courtesy Journal of the Acoustical Society of America.) 


between 60 and 140 decibels, above 2 X 10-^ 
dyne per square centimeter. 

Figure 6 shows the number of the harmonic 
on the horizontal scale, where the first har- 
monic represents the fundamental (or objec- 
tive stimulus frequency), and the intensity 
level of the fundamental and its various har- 


the harmonics are due to the nonlinear nature 
of the ear’s force-displacement characteristic, 
this result indicates that the amount of over- 
loading depends predominantly on the incident 
pressure and not on frequency. 

This figure also shows that the sensation 
levels of the harmonics may sometimes exceed 


r RESTRICTED 

\ 4 ^. 


SENSATION LEVEL WHERE HARMONICS APPEAR 


THRESHOLD FOR TONES 


27 



01 234 5678 

NUMBER OF HARMONIC 

Figure 7. Sensation levels of harmonics produced by a 50-cycle tone. (Courtesy Journal of the 
Acoustical Society of America.) 



FREQUENCY IN CYCLES 


Figure 8. 


Sensation levels required to produce aural harmonics. 


(Courtesy D. Van Nostrand Co.) 


RESTRICTED 


28 


BEHAVIOR OF THE EAR 


that of the fundamental. Consider a 50-cycle 
tone at a sensation level of 70 decibels. From 
Figure 1, such a tone has an intensity level of 
140 decibels. By comparing Figures 1 and 6, 
it will be seen that the intensities of the first 
four harmonics of a 140-decibel fundamental 
decrease at a rate of about 10 decibels per 
octave, whereas the audibility threshold be- 
tween 50 and 200 cycles falls off at a rate of 
about 15 decibels per octave. In other words, 
the sensation levels of the harmonics increase 
at a rate of about 5 decibels per octave. 

The sensation levels of the harmonics of a 
50-cycle tone have been computed in the man- 
ner just outlined, and are shown in Figure 7. 
From this figure, it is clear that, for the 
stronger stimuli, the first six overtones have 
higher sensation levels than the fundamental. 
This explains the observation^^ that the first 
harmonic of a 50-cycle tone could always be 
sensed, no matter how faint the stimulus tone 
sounded. It also explains why the subjective 
loudness of low-frequency sounds increases so 
much more rapidly with increase of sensation 
level than does the sensed loudness of higher- 
frequency tones (see Figure 11). From Figure 
8 it will be seen that the ease with which a tone 
produces aural harmonics is nearly constant 
above 1 kilocycle and increases rapidly for fre- 
quencies below this value. 

2 2 DISCRIMINATION FOR TONES 

The ear’s ability to discriminate between 
tones of different pitch and loudness helps to 
determine the effectiveness of the ear in dis- 
tinguishing a signal from the background. A 
study of the ear’s performance for tones has re- 
vealed certain simple relationships which are 
of basic importance in psychoacoustics. Since 
these relationships are helpful in understand- 
ing the results given in following chapters, 
they are stated briefly in the present section. 

Pitch 

According to the theory of hearing presented 
in Chapter 1, sounds of different frequency set 
into vibration, or stimulate, different regions 
of the basilar membrane. The ability of the 
brain to distinguish between sounds received 

% 


in different regions of the membrane then gives 
rise to perceived pitch. As noted in Chapter 1, 
this association between pitch and the position 
of maximum stimulation on the basilar mem- 
brane is called the place theory. 

Equation (5) of Chapter 1 provides an ap- 
proximate relationship between the fractional 
distance along the basilar membrane and the 
frequency of sound which produces maximum 
stimulation at that position. The values found 
from this equation are shown in Figure 9. The 
horizontal scale indicates the frequency of the 
stimulus tone, plotted logarithmically. The 
ordinate specifies the fractional distance 
(namely, x, or s/3.1) between the apex and 
base of the membrane at which the frequency 
in question produces maximal stimulation. The 
points represent the results of measurements 
obtained by techniques described in succeeding 
paragraphs. As seen, the measurements are in 
satisfactory agreement with the curve plotted 
from equation (5) of Chapter 1. 

Furthermore, the figure indicates that over 
the range between 0.5 and 10 kilocycles, the 
position stimulated is approximately propor- 
tional to the logarithm of the stimulus fre- 
quency. Because ability to distinguish differ- 
ences of pitch is a linear function of position 
on the basilar membrane, the common practice, 
which is followed in succeeding chapters, of 
plotting frequencies to a logarithmic scale is 
justified, since it provides a visual emphasis 
which approximates the auditory emphasis. 

The filled-in circles plotted in Figure 9 were 
obtained in post-mortem studies of deafened 
human ears and show the relation between the 
position of lesions among the hair cells and the 
frequency region in which the individual’s hear- 
ing was subnormal. This is the most direct line 
of evidence. 

The results of indirect evidence from the 
measured functional characteristics of human 
ears are shown by the squares obtained from 
studies of frequency discrimination and de- 
scribed later in this section, and by the tri- 
angles which represent studies of pitch bisec- 
tion. The term “pitch bisection” refers to the 
operation of finding a frequency which pro- 
duces the sensation having one-half the pitch 
evoked by a comparison frequency. Various 


/restricteb^ 


DISCRIMINATION FOR TONES 


29 


observers agree fairly well in their selections 
of the frequency which has associated with it 
a pitch sensation corresponding to half that 
produced by a standard, provided that previ- 


Figure 9 were obtained. In order to correlate 
the position of the injury with frequency, the 
magnitude of cochlear potentials produced by 
various tones were measured before and im- 



4681 2 4681 2 4681 2 4 

100 1000 10,000 


FREQUENCY IN CYCLES 


Figure 9. Position of maximum stimulation produced by different frequencies. 


ously learned relationships between musical 
intervals are disregarded. 

The open circles in Figure 9 represent mea- 
surements made upon experimental animals 
with hearing mechanisms very similar to that 
of man. Since the lengths of the basilar mem- 
branes in such animals usually differ from that 
in human ears, the measurements of position 
have been expressed as fractions of the total 
length of the membrane, so that the same verti- 
cal scale would apply in all cases. 

Animal measurements have been made in 
several ways. One method consists of finding 
the position upon the outer surface of the intact 
cochlea at which the maximum value of the 
cochlear potential occurs for an impressed tone 
of given frequency and of repeating the mea- 
surement for tones of different frequencies. 
About half of the circles in Figure 9 were 
obtained by means of this technique.^^'^® 

By carefully drilling into the cochlea and 
producing localized injury of the inner ear 
structure, the remainder of the open circles in 


mediately after the operation. In general, 
localized injury resulted in diminished response 
to tones in a fairly narrow frequency region. 
In Figure 9, the midpoints of these regions 
have been plotted against the corresponding 
position coordinates of the injuries. 

Indirect evidence confirming the validity of 
Figure 9 for human ears has been obtained by 
observations on the performance of the ear 
under different conditions. The general con- 
sistency of Figure 9 with the observations on 
masking is shown in Figure 17. 

Although Figure 9 gives the position of 
stimulation of the basilar membrane for sound 
of a given frequency, it does not indicate 
directly the least frequency difference which 
the ear can detect. This minimum perceptible 
frequency change is called the frequency limen. 
This frequency sensitivity of the ear would be 
expected to depend both on the width of the 
stimulated region on the membrane and on the 
nature of the processes by which stimulation of 
the membrane is converted into a nerve im- 




30 


BEHAVIOR OF THE EAR 


pulse and transmitted to the brain. It has been 
shown in Chapter 1 that the vibration of the 
strings in the basilar membrane is transmitted 
to the auditory nerve by means of the hair cells. 
It may be anticipated that no difference in fre- 
quency between two successive tones can be de- 
tected if it corresponds to a shift of position 
along the membrane by less than the spacing 
between adjacent groups of cells, that is, by 
less than the spacing between the Corti arches. 
Since there are about 6,000 of these members 
in the ear, the frequency limen may be expected 


sation level of the tone used was about 40 deci- 
bels ; the results do not depend critically on the 
sensation level for higher levels. It is neces- 
sary, of course, that the two tones to be differ- 
entiated be presented successively. If presented 
simultaneously, even a slight frequency differ- 
ence would give rise to beats which could 
readily be heard. 

The data in Figure 10 permit the computa- 
tion of the smallest number of frequency steps 
between the lowest and highest audible fre- 
quencies (about 1,500). Assuming that each 


100 


10 


I 



10 100 1000 10,000 


FREQUENCY IN CYCLES 

Figure 10. Frequency limen for different frequencies at a sensation level of 40 decibels. 


to correspond to a change of x by at least 
1/6,000 or 1.67 X 10“^. If the stimulation of the 
membrane is distributed over a very wide 
region, even greater differences in frequency 
might go undetected however. 

The observed values^® for the frequency 
limen If are given by the plotted points in 
Figure 10. In these tests, the frequency of a 
sustained tone was shifted up and down, about 
3 times a second, over a range A/ cycles. The 
values of A/ for which the warbling effect could 
just be distinguished are plotted in the figure. 
With somewhat different methods of presenta- 
tion about the same values were obtained,^® 
although with faster or slower frequency 
sweeps the ear became less sensitive. The sen- 


step requires a shift of the point of maximum 
stimulation on the basilar membrane by a fixed 
distance gives the squares shown in Figure 9. 

The smooth curve drawn in Figure 10 was 
obtained by differentiating equation (5) in 
Chapter 1. It will be seen that the agreement 
between the theory and the observations is good 
for frequencies above 400 cycles per second. 
Disagreement for the lower frequencies is be- 
lieved^® to arise from the aural harmonics 
which these tones so readily generate. 

Figure 10 indicates that the stimulated 
region of the membrane is relatively narrow 
when a single tone is present. However, the 
evidence on the masking of tones by tones, 
presented in the previous section, suggests that 


( RESTRICT^ ] 


DISCRIMINATION FOR TONES 


31 


this region is relatively wide. The data on the 
masking of tones by wide-band sounds, dis- 
cussed in Section 2.3, indicate that a pure tone 
sets into vibration a segment of the basilar 
membrane whose relative width Ax is about 
20 times that used in computing the curve 
shown in Figure 10. It is not clear how the 
ear can determine so accurately the center of 
the excited region. Various suggestions along 
this line are discussed in reference 17. Although 
the theoretical basis of Figure 10 is not wholly 
clear, this figure gives a reliable indication of 
the frequency sensitivity of the ear under ideal 
conditions. 

Loudness 

The subjective loudness of a sound is ap- 
parently closely related to the masking proper- 
ties of the sound and to the detectability of the 
sound in the presence of masking backgrounds. 
In addition, the ability to detect one sound in 
the presence of another depends on the loud- 
ness of each sound separately. Ability to de- 
tect small changes of loudness under various 
conditions is of crucial importance in listening 
for modulated sounds. Thus a study of loudness 
and loudness discrimination is important to the 
specific subject of underwater sound detection 
as well as to psychoacoustics in general. The 
meaning of loudness is discussed in the follow- 
ing paragraphs, while loudness discrimination 
is treated at the end of this section. 

Measurement of Loudness 

Since the ear is not a quantitative measuring 
instrument, it is not possible to measure di- 
rectly the subjective loudness of a sound. The 
observer can, at most, state that one sound has 
greater loudness, the same loudness, or less 
loudness than another sound. Since the ear can 
identify two sounds as having the same loud- 
ness, it is at least possible to adjust the rela- 
tive intensities of widely different sounds until 
they have the same loudness. Both systems of 
measuring loudness which are now in use start 
from this basic fact. Fortunately most observ- 
ers with normal hearing usually agree with 
each other as to whether or not one sound is 
equal in loudness to another. 


The simplest measurement of loudness is to 
refer all sounds to a sustained tone of 1,000 
cycles per second. The loudness of any sound 
may then be measured in terms of the intensity 
level of the 1-kilocycle tone of equal loudness; 
this is equal to the sensation level of the 1-kilo- 
cycle tone (see page 17). The level so de- 
termined is called the loudness level of the 
sound. The phon is used for giving the loud- 
ness level. A sound whose loudness is the same 
as that of a 1-kilocycle tone at an intensity 
level of 50 decibels has a loudness of 50 phons. 
In general, a loudness of 70 phons is regarded 
as a comfortable listening level for typical 
sounds. Sustained tones of high pitch at this 
level can become very annoying. 

Two tones of different frequencies but of the 
same loudness level will usually have different 
intensity levels. All tones at the threshold of 
audibility have zero loudness. It is not true, 
however, that all tones at the pain threshold 
are equally loud. 

Equal loudness contours are shown in Figure 
11, where the intensity levels for sounds of 
constant loudness are plotted against fre- 
quency. The thresholds of audibility and pain 
correspond, of course, to those shown in Figure 
1. Since the contours approach each other at 
very low frequencies, it is evident that given 
changes of loudness correspond to smaller in- 
tensity changes at the very low frequencies. 
This effect is presumably owing to the larger 
stimulation contributed by aural harmonics of 
low-frequency tones (see Figure 8). 

Another method of measuring loudness has 
been developed which presumably has more 
objective significance. In this method, it is 
assumed that the loudness of a sound heard 
with two ears is equal to twice the loudness of 
the same sound heard with only one ear. On 
the basis of this assumption, the observed data 
can be used to draw a curve for each frequency 
connecting the loudness with the intensity level. 
Other similar assumptions lead to much the 
same results, and there is some reason to be- 
lieve that these methods do in fact measure 
subjective loudness. Since these results have 
not as yet been applied to the study of the 
recognition of underwater sounds, they are 
only mentioned in passing here. 


RESTRICTED 




32 


BEHAVIOR OF THE EAR 


Loudness of Pulses 

The data summarized in Figure 11 refer to 
the loudness of sustained tones. Some work has 
also been carried out by the Bell Telephone 
Laboratories on the perceived loudness of short 
pulses of varying duration. These results have 
been made available in advance of publication 
(see also reference 22), and are reported here 
because of their relevance to the masking of 
short pulses by noise and reverberation. 


level of the hypothetical tone of equal intensity. 
In this way, for each of the five pulse lengths 
used a curve was drawn showing the loudness 
level of the pulse as a function of the loudness 
level of the equally intense sustained tone of 
the same frequency. The tests were carried out 
for three frequencies, 125, 1,000, and 5,650 
cycles per second. The resulting plots are given 
in Figures 12, 13, and 14. 

It is evident from these figures that when the 



100 1000 10,000 
FREQUENCY IN CYCLES 


Figure 11. Equal-loudness contours. Loudness levels in phons are indicated on curves. (Courtesy 
Journal of the Acoustical Society of America.) 

In these tests the intensity level of a short 
pulse was varied until its subjective loudness 
was the same as that of a sustained tone at the 
same frequency and of known loudness level. 

A value was then read from Figure 11 for the 
loudness which the sustained tone would have 
if its intensity were just equal to that of the 
pulse. The measured loudness level of the pulse 
was then plotted against the computed loudness 


loudness level of the tone is appreciable, a re- 
duction in the duration of the presented tone 
results in a marked decrease of the subjective 
loudness. This is a result of what has been 
called the finite build-up time of the ear. Other 
evidence also indicates that when the duration 
of a sound is less than second, the effective- 
ness of aural perception diminishes, and the 
subjective loudness decreases. 


{ RESTRICT^ 



DISCRIMINATION FOR TONES 


33 


Figures 12 through 14 seem to show, how- 
ever, that as the auditory threshold is ap- 
proached, the finite build-up time of the ear 
seems to lose its effect. A tone which is barely 



Figure 12. Loudness levels of tonal pulses (125 
cycles). (Courtesy Bell Telephone Laboratories.) 


shown on the horizontal scale.' The rate of am- 
plitude modulation used in these tests amounted 
to 3 cycles per second. The vertical scale at the 
right gives the minimum intensity change in 



Figure 13. Loudness levels of tonal pulses (1,000 
cycles). (Courtesy Bell Telephone Laboratories.) 


audible when sustained, in the absence of any 
masking background, can apparently still be 
heard when its duration is reduced to as little 
as 0.05 second. While the curves are obviously 
extrapolated for very low loudness levels, the 
general nature of this extrapolation seems well 
indicated by the data. This result is presumably 
of considerable psychoacoustic significance in 
that it casts important light on the properties 
of the ear’s build-up time. 


Loudness Discrimination 

When two tones with slightly different loud- 
ness are presented successively, or when the 
intensity of a tone is varied, the change of loud- 
ness can be detected only if it is greater than a 
certain value. The minimum perceptible inten- 
sity increment, which depends on the rate of 
modulation, the sound frequency, and the loud- 
ness level, is called the intensity limen. For 
young observers with normal hearing, mea- 
sured values^® of the intensity limen are shown 
in Figure 15 for all values of the frequency 


decibels, while the scale on the left gives directly 
the relative intensity change A///. 



0 20 40 60 80 100 120 


LOUDNESS LEVEL OF 5650- CYCLE TONE IN PHONS 

Figure 14. Loudness levels of tonal pulses (5,650 
cycles). (Courtesy Bell Telephone Laboratories.) 

Figure 16 shows the dependence of the in- 
tensity limen on the rate of amplitude modula- 


RESTRICTED 


34 


BEHAVIOR OF THE EAR 


tion of a 1-kilocycle tone, for loudness levels of 
25 and 50 decibels. It indicates that intensity 
discrimination improves with intensity. About 
the same results would probably be obtained 
at frequencies between 0.6 and 10 kilocycles. 
For example, results similar to those in Figure 
15 were obtained in a study with a wide band 
of thermal noise.^® 


of whatever acoustic character. Hence it is 
usually best to distinguish between background, 
or unwanted sounds, and signals, or wanted 
sounds. 

When a complex sound, with components dis- 
tributed over a large band of frequencies, is 
incident upon the ear, the latter continues to 
act as an analyzer. Thus, it is possible to focus 



16 32 64 128 256 512 1024 2048 4096 8192 16384 

FREQUENCY IN CYCLES 


AI/I 
IN DB 


Figure 15. Intensity limen at different frequency and sensation levels. (Courtesy Physical Review.) 


COMPLEX SOUNDS attention upon different pitch regions in such 

a sound, and the effect of removing some of its 


By a complex sound is meant a pressure dis- 
turbance with a complex acoustic spectrum. 
Thus, at one extreme is a pure tone, correspond- 
ing to a line spectrum. An intermediate case is^ 
illustrated by a musical chord, or even by the 
sound obtained when all the keys of a piano 
keyboard are struck simultaneously, giving a 
very complicated line spectrum. At the other 
extreme is the type of sound produced by break- 
ing surf, containing all possible frequencies 
within the sonic band. 

The term noise is often used to denote a com- 
plex sound, and many, but not all, common 
noises are of this character. However, the term 
noise is also used to mean an unwanted sound 


1.0 

.8 

.6 

AI/I 

.4 

.2 

0 


Figure 16. Dependence of the intensity limen 
on the rate of variation in the intensity of a 1- 
kilocycle tone. The open circles refer to a sensa- 
tion level of 25 decibels and the filled-in circles 
to a sensation level of 50 decibels. (Courtesy 
Physical Review.) 










\ 







r 








r 


















2 .4 .8 1.6 3i2 6.4 12^ 25.6 51.2 

NUMBER OF BEATS PER SECX)ND 


RESTRICTED^ 


COMPLEX SOUNDS 


35 


frequencies by electrical or acoustic filtering 
is quite perceptible. 

The masking and loudness produced by com- 
plex sounds are in general simpler to predict 
than are the corresponding properties for indi- 
vidual tones or groups of tones. In the case of 
a single tone, a large part of the incident energy 
goes to stimulate a region on the basilar mem- 


quency in question. In other words, if the dis- 
tribution of energy among the various frequen- 
cies in the objective sound is not so skewed that 
the aural harmonics generated by the low-fre- 
quency content have higher sensation levels 
than are produced by the objective components 
at those frequencies, then all the masking of 
any consequence will be adjacent masking. As 



10 100 1000 10,000 
FREQUENCY IN CYCLES 

Figure 17. Aural critical bands for signal-background mixtures presented to both ears. 


brane tuned to the stimulus frequency, but 
some portion of the incident energy is also scat- 
tered to other frequencies because of the non- 
linear character of the ear’s response. The 
resultant masking and stimulation is therefore 
strongly dependent on the harmonics, as well as 
on combination and difference frequencies, 
introduced by the hearing mechanism. 

In the case of complex, or wide-band sounds, 
the scattering of energy to portions of the 
basilar membrane not tuned to the stimulus 
frequency occurs for each of the components 
in the stimulus. In any one region about as 
much sound will usually be scattered out as is 
scattered in. Thus the net result of this process 
of multiple scattering is that the stimulation 
at any point on the membrane is determined 
essentially by the fraction of the energy in the 
complex sound contained in a narrow frequency 
band, called a critical band, centered at the fre- 


Figure 6 indicates, the levels of harmonics fall 
off quite rapidly even when the fundamental is 
at the pain threshold. For ordinary listening, 
therefore, it is not likely that complex back- 
grounds will produce remote masking unless 
the energy of the various components decreases 
more rapidly with increasing frequency than 
about 20 to 30 decibels per octave. 

Just as all the masking produced by complex 
sounds is essentially adjacent masking, so also 
the stimulation and loudness they produce can 
be computed directly from the objective spectra, 
without regard to the harmonic content intro- 
duced by the ear, unless the complex sounds 
contain salient peaks or drop off at rates in 
excess of 20 decibels per octave. Subject to this 
restriction, the loudness of a complex sound is 
generally the sum over all critical bands of the 
loudness in each band. 

The facts concerning masking by distributed 


( ^STRICTED^ 


36 


BEHAVIOR OF THE EAR 


sounds have been determined in the following 
^ray 20.21 ^ wide-band sound (thermal noise in 
the case studied), with a flat spectrum extend- 
ing from, say, 400 to 1,200 cycles, is presented 
to the ear as a constant-level background. Then 
the level of an 800-cycle signal which is just 
audible in the presence of this background is 
determined. Successive redeterminations of 
this signal level are made, keeping all the con- 
ditions the same as already described, with the 
exception that a band-pass Alter centered at 
800 cycles is introduced to eliminate some of 
the background components. 

Since, as stated before, remote masking is 
not very significant in the case of wide-band 
sounds, the level of the just detectable 800-cycle 
signal is not affected by removing the upper 
and lower frequencies from the background 
band. This constancy of the level of the just 
detectable 800-cycle signal continues to be ob- 
served as the band of admitted backgi'ound fre- 
quencies is narrowed, until a critical width of 
the filter band is reached. For an 800-cycle 
tone masked by distributed noise, this critical 
band width amounts to 40 cycles (see Figure 
17). For narrower bands of noise (centered at 
800 cycles), the level of the just audible 800- 
cycle tone diminishes in direct proportion to 
the decrease in the band width of the back- 
ground ; that is, when the band is cut from 40 
to 20 cycles, the intensity of the just audible 
signal drops by a factor of 2. 

Similar experiments, conducted for tones of 
other frequencies, give the widths of critical 
bands centered at different points in the sonic 
range of frequencies, and these are also shown 
by the plotted points in Figure 17. Thus, in the 
presence of distributed sounds, the ear behaves 
as though it were provided with a group of 
band-pass filters which permit it to eliminate 
masking interference from all components 
beyond the cut-off limits of the filters. The tests 
show, furthermore, that a sustained tonal sig- 
nal is just audible when its intensity is equal 
to the intensity of the background components 
contained in the critical band centered at the 
tone frequency. 

Evidence discussed in Chapter 4 indicates 
that the critical band criterion also applies 
when the signal is a distributed sound. Under 
these circumstances, the signal becomes de- 


tectable in the presence of a distributed back- 
ground when the intensity of the signal equals 
the intensity of the background in at least one 
critical band. However, fluctuations of signal 
and background level may modify the latter 
result in certain cases (see Figures 29, 38, and 
40 in Chapter 4). 

The width of a critical band is determined 
essentially by the fact that even when the ear 
is stimulated by a pure tone of low intensity 
the basilar membrane vibrates in patches of 
width A5. This width has been estimated in 
Section 2.1.2 as being about 3 X lO-^ centimeter, 
or 1 per cent of the total extent of the mem- 
brane. This same result may be obtained from 
the critical band width A/ given in Figure 17 
and the curve in Figure 9. By finding the value 
of Ax on the ordinate of Figure 9 which cor- 
responds to the value of A/ on the ordinate of 
Figure 17, for a given value of the frequency, 
it will be seen that the value of Ax correspond- 
ing to one critical band is very nearly equal to 
0.01, that is, 1 per cent, for all frequencies. 
Since x=s/3.1 (see Section 1.2), Ax=As/3.1; 
hence, when Ax is 0.01, As amounts to 3X10“* 
centimeter. 

With this value of 0.01 for Ax, values of A/ 
have been computed. The computation is ele- 
mentary, since A/ should equal df/dx times Ax, 
where df/dx may be obtained either by differ- 
entiating equation (5) of Chapter 1 or from 
the graph in Figure 9. The values found in this 
way are shown by the smooth curve in Figure 
17. It is evident that the agreement is close. 
This figure may be compared with Figure 10, 
showing the corresponding agreement between 
predicted and theoretical values of the fre- 
quency limen. This comparison shows that the 
critical band width A/ at each frequency is 
about 20 times the frequency limen at the same 
frequency. This general agreement between ob- 
servation and theory strengthens confidence in 
the concepts developed in this chapter and the 
preceding one and indicates that these concepts 
can be reliably applied to a tentative interpre- 
tation of practical data. However, it is evident 
that other factors, such as the nature of nerve 
conduction and the build-up time of the ear, are 
not as yet wholly understood and may be ex- 
pected to affect practical results in unforeseen 
ways. 


i RESTRICTED 


Chapter 3 

CHARACTERISTICS OF TARGET SOUNDS AND NOISE BACKGROUND 


T he study of underwater sound detection, 
or recognition, may be divided into two 
parts, depending on the type of sound equip- 
ment and how it is used. Sound gear may be 
used to detect the sound radiated by some ob- 
ject and is then called listening gear. It may 
also be employed to detect sounds first radiated 
by the gear and then refiected back from some 
object; such installations are described as echo- 
ranging gear. The object which either gener- 
ates or refiects the desired sound is often called 
the target, and the signal which it directs to- 
ward the gear is known as either the target 
sound or the echo, depending on its origin. Ob- 
viously, echo-ranging gear can be used as lis- 
tening gear, and in current practice the oper- 
ator alternates both these uses. Chapters 4 
through 6 discuss the recognition of target 
sounds, while Chapters 7 through 11 discuss 
the recognition of echoes. 

The hydrophone which receives the acoustic 
energy is usually an electromechanical con- 
verter which transforms the sound impulses 
into equivalent electric impulses. These elec- 
tric impulses are subsequently amplified and 
may be changed in other ways which affect 
overall performance. Some of these changes 
will be discussed in this chapter. The output 
of the hydrophone and its associated circuits 
is finally coupled to the mechanism used to 
detect the signal; the ear, together with head- 
phones or a loudspeaker, is the detector con- 
sidered throughout this volume. 

The major factors determining the possibil- 
ity of signal detection are the characteristics 
of the received signal and background, the 
properties of the gear, and the limitations of 
the detector. These factors will be discussed 
in sequence in this chapter. Chapters 4 and 5 
describe the data obtained by different groups 
on the recognition of target sounds. 

The character of the sounds received by the 
gear and presented to the operator is affected 
by a number of factors, few of which are sub- 
ject to control. The typical signal discussed in 


Chapters 4 and 5, acoustic radiation from a 
surface ship or a submarine, depends among 
other things on (1) the orientation of the tar- 
get vessel relative to the listening vessel, (2) 
the type of target vessel, (3) its speed, and 
(4) the operating condition of its machinery. 
The intensity, frequency composition, and 
phase relations of the radiated energy may all 
be modified during transmission from source 
to receiver.^ 

The amount and character of the interfering 
background depend on (1) the nature of the 
sounds produced by other sources, such as 
waves and whitecaps; (2) how well those 
sounds are transmitted to the hydrophone and 
to the ear; and (3) the existence of nonacous- 
tic disturbances, such as electric fields pro- 
duced by motors, which may be transformed 
into sounds by the various parts of the listen- 
ing gear. Airborne sounds not received through 
the listening gear may also form part of the 
interfering background, although the effect of 
these noises can usually be minimized by in- 
creasing the amplifier gain. 

Many listening hydrophones are designed to 
respond efficiently only to sounds whose angles 
of incidence with the hydrophone face lie 
within a fairly narrow cone. The axis of this 
cone is called the hydrophone axis. This abil- 
ity of hydrophones to respond preferentially to 
sounds incident on the hydrophone axis is 
termed hydrophone directivity, and hydro- 
phones which possess this property are called 
directional hydrophones. Hydrophone directiv- 
ity increases as the size of the hydrophone 
increases and as the frequency of the incident 
sounds increases. Hydrophone directivity 
serves two functions: (1) it diminishes the 
amount of interfering background transmitted 
to the operator, wherever such interference 
reaches the hydrophone from directions that 
are not on its axis; and (2) it furnishes a 
method for determining the relative bearing 
of the target. Additional discrimination of the 

^ See STR Division 6, Volume 8. 


37 






RESTRICTED 


38 


CHARACTERISTICS OF TARGET SOUNDS AND NOISE BACKGROUND 


receiver against various frequencies present in 
the background may be obtained by designing 
the gear to transmit some frequencies present 
in the background and to suppress others. This 
procedure may be undesirable since it sup- 
presses the same frequencies in the signal; 


of these various factors, a more detailed de- 
scription of the characteristic features of ship 
signals and interfering sounds is given in Sec- 
tions 3.1 and 3.2. Those features of the listen- 
ing gear which affect the recognition of target 
sounds are described in Section 3.3. 



STEAM FREIGHTER, 260 BEATS PER MINUTE 
UNDERWATER NOISE IN BAND BETWEEN I AND 8.5 KC 



DIESEL FREIGHTER, 248 BEATS PER MINUTE 
UNDERWATER NOISE IN BAND BETWEEN I AND 8.5 KC 



LARGE FREIGHTER, 8.5 KNOTS, 268 BEATS PER MINUTE 
UNDERWATER NOISE IN BAND BETWEEN I AND 8.5 KC 



OLD DESTROYER, 15 KNOTS 
SUPERSONIC NOISE IN 3.5-KC 
BAND CENTERED AT 24 KC 
PEAK LEVEL IS ABOUT 16 DB ABOVE 
RMS LEVEL 


Figure 1. Oscillograms of sonic and supersonic ship noise. The playback of the signal was applied 
to the terminals of a nonpersistent CRO, and the resulting deflections were photographed on con- 
tinuously moving film. 


thus, it may discriminate against precisely 
those signal frequencies which permit ready 
detection of the target. As shown in Chapters 
1 and 2, the ear possesses its own frequency- 
discriminating mechanism. 

In order to evaluate the effects upon the ear 


3 1 SOUNDS FROM SUBMARINES AND 
SURFACE VESSELS 

The character of the acoustic energy radi- 
ated into the water by a submarine or sur- 
face vessel is modified during transmission. 



SOUNDS FROM SUBMARINES AND SURFACE VESSELS 


39 


The nature of the signal at the listening hydro- 
phone, compared with that of the masking 
background, is what determines whether the 
target will be detected. Unfortunately, accu- 
rate measurements of this signal are difficult 
to make at distances comparable to the maxi- 
mum distances at which listening gear can de- 
tect the source vessel, because the signal-to- 
noise ratio is quite unfavorable for most mea- 
suring devices at such long ranges. Conse- 
quently, the acoustic outputs of ships are usu- 
ally measured at ranges of 50 to 500 yards, 
and the probable characteristics of the radi- 
ated sound at practical detection distances are 
deduced from fundamental studies on the trans- 
mission of underwater sound. 


Close to Source 

While ship sounds are often intense, they 
never represent more than a tiny fraction of 
the energy lost by the ship through various 
kinds of operating inefficiency. They are there- 
fore not necessarily diminished by general im- 
provements in design ; specific attention to 
their causes is required in order to reduce their 
intensity. The radiated energy is acoustically 
complex; it is distributed throughout the spec- 
trum and is appreciable even in the subsonic 
and supersonic regions. There are two major 
sources of sound : the propellers and the engine 
room. 

Most of the sound coming from the propel- 
lers is produced by cavitation, that is, from 
the formation of cavities in the water near the 
propeller blades. These cavities are not due 
to the churning of air into the water; they 
are formed when the pressure behind the mov- 
ing blades falls to a threshold value, which 
increases with increasing hydrostatic pressure. 
The rapid pressure changes which accompany 
the formation and collapse of these cavities 
constitute the radiated sound. When such cavi- 
ties collapse against the blades, the impact 
is large; this effect probably accounts for the 
high rate of propeller erosion. 

In practice, the amount of cavitation noise 
is determined by (1) propeller tip speed; (2) 
acceleration, including the application of helm ; 
(3) loading, in the case of cargo vessels and 


transports; and (4) the depth of the propeller 
below the surface. By way of an example of 
the last point, submariners engaged in evasive 
maneuvers can diminish the amount of cavita- 
tion noise radiated by the submarine by oper- 
ating at increased depth, although there is 
some speed, at any practical depth, which 
marks the onset of cavitation. For a given 
speed, cavitation noise is greater on the aver- 
age for vessels of larger tonnage. 

Cavitation sounds represent the total effect 
of a large number of independent events. Each 
of these events, the formation or collapse of a 
cavity, generates a brief pressure pulse with 
a complex acoustic spectrum. When cavitation 
noise is subjected to frequency analysis, it is 
found that, the intensity within a narrow band 
is very nearly inversely proportional to the 
square of the frequency, for frequencies be- 
tween 100 and 30,000 cycles per second.’’ In 
other words, the spectrum has a negative slope 
of 6 decibels per octave [10 log (f/2f)- = — 6 
decibels]. The steep decline of intensity with 
frequency does not necessarily mean that the 
low frequencies are the most important from 
the standpoint of detectability, because detecta- 
bility is determined by the relative strengths of 
signal and background at various frequencies. 
If desirable, the high frequencies may be am- 
plified more than the low. 

To the ear, cavitation is a rushing, boiling 
sort of sound without much character. What 
character cavitation sounds have is conferred 
by propeller modulation. Very often, all the 
frequencies in the observed cavitation sound 
are amplitude-modulated at the blade rate or 
the shaft rate or both. The percentage modula- 
tion which is observed depends on the type of 
vessel and the condition of its operation (see 
Figures 1 and 2), and is probably one of the 
factors which enables an experienced sound 

These frequencies are at present the practical upper 
and lower limits for most types of underwater sound 
gear; in a few applications, however, frequencies out- 
side these limits are used. An upper limit is imposed 
by the increase in transmission loss with increasing 
frequency; a lower, by the increase in background in- 
terference with decreasing frequency as well as by the 
properties of the ear (see Section 2.1.1). Ship spectra 
do not continue to rise at the very low frequencies; 
there is often a peak in the neighborhood of 30 to 50 
cycles per second and an abrupt decline for still lower 
frequencies. 


RESTRICTED 4 


40 


CHARACTERISTICS OF TARGET SOUNDS AND NOISE BACKGROUND 


operator to classify the target. Well-modulated decibels higher than the spectrum level of dis- 
propeller cavitation has been described as hav- tributed sounds at neighboring frequencies, 
ing a characteristic rasping sound. The remainder of the spectrum is typical of 



Figure 2. Oscillograms of submarine and torpedo noise in the band between 1 and 8.5 kilocycles. 
The playback of the signal was applied to the terminals of a nonpersistent CRO, and the resulting 
deflections were photographed on continuously moving film. 


The blade and shaft rate modulations are 
often quite distinct in character, and when 
both are present the propeller sounds usually 
have an accented beat, so that every third or 
fourth “chug” is louder than the intervening 
ones. For multi-screw vessels, there are addi- 
tional possibilities for complicated rhythm pat- 
terns. The accent of propeller sounds is often 
characteristic of a particular ship and provides 
an additional method of identifying the source. 
A count of the number of propeller beats per 
minute is useful in estimating the speed of a 
target; when the rate is too high for comfort- 
able count of the individual beats, it is helpful 
to count the accented beats only, if the beat is 
accented, and to multiply by the number of 
beats in the rhythm cycle. 

In addition to cavitation noise, propellers 
may produce more nearly tonal kinds of sound. 
Improperly designed propellers often show 
characteristic torsional or flexural vibrations 
which produce very intense tones. Figure 3 
shows the spectrum of a ship with a “singing” 
propeller. The single-frequency peak, shown 
as a vertical line at 1,100 cycles, is about 35 


cavitation, and has a slope of about 6 decibels 
per octave. The 1,100-cycle tone observed in 


90 r— I 1 —^ 16 



I 2 4681 2 4681 


100 1000 10,000 
FREQUENCY IN CYCLES 

Figure 3. Spectrum of an aircraft carrier at 15 
knots, measured 220 yards from the source. 

this case was heard as a whine which was 
amplitude-modulated at the propeller rate, 
showing that it originated at the screws. 


SOUNDS FROM SUBMARINES AND SURFACE VESSELS 


41 


Finally, propeller shafts occasionally pro- 
duce an assortment of characteristic moans, 
squeaks, and howls. These sounds are particu- 
larly likely to occur as the result of damage 
sustained in action, and the increased noise 
constitutes an additional operating hazard until 
repairs can be made. 

Obtaining the spectrum of a noise source 
consists, essentially, of measuring the average 
energy contained in narrow frequency bands 
at various frequencies. As shown in Chapter 
2, adequate resolution of the spectrum (from 
the point of view of loudness and masking) 
may be obtained by analyzing the sound with 
band-pass filters about 50 cycles wide. When 
a distributed sound is measured in this way, 
the total energy passed by the filter increases 
with the width of its pass band. If a pure tone 
is measured through a filter which passes the 
tone frequency, the measured energy is inde- 
pendent of filter width. Therefore, the in- 
tensity of a pure tone mixed with a distributed 
sound may be computed by noting the differ- 
ence between the reading obtained when the 
filter passes the tone together with the distrib- 



Figure 4. Spectrum of a fleet-type submarine 
at 50 rpm (2.5 knots), and periscope depth, mea- 
sured 150 yards from the screws. 


uted sound, and the reading obtained when 
the filter passes only the distributed sound at 
frequencies in the immediate neighborhood of 
the tone. Alternatively, two filters of different 


widths may be used to pass the mixture, and 
the contribution due to the tone may be esti- 
mated by the method described in Section 4.1.3. 
Sounds entering the water from the interior 



Figure 5. Spectrum of a fleet-type submarine at 
120 rpm (6 knots), and periscope depth, mea- 
sured 170 yards from the screws. 


of the vessel generally originate at either the 
propulsion or the auxiliary machinery. Some 
of these sounds increase in intensity with in- 
creasing speed ; others are independent of speed 
and may persist even when the vessel is adrift. 
Where such machinery is not mounted on an 
insulating base, most of the sound reaches the 
water by conduction through the supporting 
structures. However, even when vibration 
mounts are used, some energy reaches the hull 
by air conduction, and the transmission of this 
fraction can be reduced only by soundproofing 
the walls. 

The spectra of machinery sounds have re- 
ceived little study, but several general remarks 
appear warranted. Such spectra may exhibit 
a group of fairly intense, harmonically related 
tonal components. This property is character- 
istic of many kinds of rotating machinery, 
such as reduction gears, ventilating fans, and 
electric generators. Such tonal components are 
about equally strong at frequencies below 1 
kilocycle, diminish in intensity at higher fre- 
quencies, and are either very weak or rela- 
tively rare at frequencies above 3 kilocycles. 
Figures 4 and 5 illustrate this type of spectrum. 
Such tones often sound like the whine of a buzz 
saw and are usually frequency-modulated and 
amplitude-modulated. Occasionally, as in the 
case of reduction gear noise produced by sub- 


42 


CHARACTERISTICS OF TARGET SOUNDS AND NOISE BACKGROUND 


marines, they increase in frequency with in- 
creasing speed. Another type of machinery 
spectrum, heard as a grinding or rumbling 
sound, is illustrated in Figure 6. In general, 
machinery spectra appear to be rich in low- 
frequency components; they tend to be rela- 
tively flat in the region below 1 to 2 kilocycles, 
and to drop off at a rate considerably in excess 
of 6 decibels per octave at higher frequencies. 
The average spectrum slope of many typical 
machinery sounds seems to be about —12 deci- 
bels per octave in the region above 2 kilocycles. 
Therefore, most machinery sounds are too faint 
at supersonic frequencies to be detected with 
supersonic listening gear.'" The only common 
exception to this observation is the type of 
sound produced by machines used to move 
fluids, such as pumps, compressors, or diesel 
exhausts. Such sibilant or explosive sounds 
are often fairly intense at frequencies well 
above 2 kilocycles and occasionally have ap- 
preciable supersonic content. Exhaust noise 
resembles automobile backfire. When diesel ex- 
hausts vent below the surface, the intensity of 
the underwater sound received from this source 
is very much greater than when they vent in 



FREQUENCY IN CYCLES 


Figure 6. Spectrum of an S-class submarine at 
anchor, charging batteries with diesels. 

air. Properly designed silencers may minimize 
the total output, however. 

Finally, there is a miscellaneous group of 
noise sources which are occasionally important. 

® In supersonic listening gear the frequency of the 
received sounds is reduced by heterodyning, so that the 
signal presented to the operator lies in the audio- 
frequency region. 


This group includes (1) the slapping of waves 
against the hull, which is a function of sea 
state, course, and speed; (2) the bow wave, 
which is a function of hull contour and speed ; 
(3) “body rumble,” which arises from the rat- 
tling of plates and other loose structures, and 



Figure 7. Equal-intensity contours for a 14-knot 
destroyer, measured in the band between 200 and 
400 cycles. The contours give pressure levels in 
decibels relative to the maximum level observed 
beneath the ship; distances are measured in feet 
in the horizontal plane. 

from the fact that a ship’s hull is a drum which 
is subjected to various kinds of percussion; 
and (4) crew activities, such as voices, foot- 
steps, hammering, and the ringing of gongs. 
This miscellaneous group generally produces a 
very small part of the total output, but it is 
important in a number of ways. Noise from 
crew activities, for example, may endanger a 
submarine taking evasive action ; similarly, 
body rumble of the listening vessel may ham- 
per sonic detection of targets. 

All these various sources are generally dis- 
tributed over the surface and through the in- 
terior of the hull. The underwater sound field 
near a ship generally changes with the bearing 
of a measuring hydrophone as well as with its 
distance from the ship. Figure 7 shows con- 
tours of equal intensity for sound in the 200- 
to 400-cycle band produced by a destroyer 
under way. Acoustic shadows cast by the hull 
and the wake would be expected to have pre- 
cisely this eifect on the contours of radiated 
sounds; such shadows should be better defined 
at high frequencies than at low. This effect is 
shown in Figure 8, which gives the contours 
measured near a moving freighter in the 2,500- 
to 5,000-cycle band. The contours in Figure 8 


SOUNDS FROM SUBMARINES AND SURFACE VESSELS 


43 


are centered astern and presumably represent 
radiation predominantly from the propellers; 
contours of engine room noise tend to be cen- 
tered amidships, as in Figure 7. In general, 
the intensity of propeller noise measured at 


60 " 90 " 120 " 



Figure 8. Equal-intensity contours for an 8-knot 
freighter, measured in the band between 2.5 and 
5 kilocycles. The contours give pressure levels 
in decibels relative to the maximum level ob- 
served beneath the ship; distances are measured 
in feet in the horizontal plane. 


quency composition, and time pattern (changes 
in the signal with time). Intensity and com- 
position are shown by the spectrum of the sig- 
nal, but spectra usually represent time aver- 
ages and do not reveal time patterns present 
in the signal. The time patterns in ship signals 
are essentially variations of intensity, fre- 
quency, and phase. The ear may be very sensi- 
tive to variations in any of these, and to audi- 
tory motion in general. It was, for example, 
indicated in Section 2.2.2 that the ear is most 
sensitive to intensity changes occurring at a 
regular rate of about three per second, and that 
it is less sensitive to slower or more rapid 
changes. The several kinds of auditory motion 
may occur singly or in a great variety of highly 
individual combinations. Figure 56 in Chapter 
4, for example, depicts the time pattern in cavi- 
tation noise produced by a submarine at peri- 
scope depth. This pattern is due to propeller 
modulation and has a period of about 2 beats 
per second. Figure 45 in Chapter 4 shows 
amplitude modulation in a tonal component in 
the spectrum of a tanker. These traces repre- 


a given distance from the propeller is greater 
for a receding than for an approaching ship. 
This dependence of intensity and composition 
on aspect (see Figure 9), together with 
changes in the intensity of cavitation due to 
acceleration, often enables sound operators to 
tell when a vessel changes course. Similarly, 
a change in the number of propeller beats in- 
dicates a change of speed. 

Thus the acoustic outputs of ships are mix- 
tures of sounds, and these mixtures can be put 
together in a large number of distinctive and 
easily recognizable combinations. Sound oper- 
ators with experience in a particular harbor 
have been known to baffle the uninitiated 
through their uncanny ability to describe the 
superstructures of approaching ships by mere 
listening. This capacity of listening gear helps 
to classify unseen or poorly visible sources, 
since the resemblance in output between two 
sources of the same type usually outweighs 
individual differences, and it gives such gear 
obvious practical advantages over detectors 
which cannot identify sources. 

The chief properties of the signal which help 
to detect and identify it are intensity, fre- 



Figure 9. Spectra of an aircraft carrier mea- 
sured at various aspects. Beam measurements 
were made 120 yards from the source; others 
were made at a distance of 240 yards. 

sent variations in rms level in a band of indi- 
cated width, as measured by a power level 
recorder with a high writing speed. It is evi- 
dent that the mean value of the rms level may 
differ by several decibels from the maximum 


RESTRICTED 



44 


CHARACTERISTICS OF TARGET SOUNDS AND NOISE BACKGROUND 


and minimum values of rms level. As shown 
in Sections 4.2.3 and 5.1, detectability may often 
be determined by maximum and minimum 
rather than average values of the rms level. 


The successive vertical bands are propeller 
modulations occurring at a rate of about 3 
per second; the nearly continuous horizontal 
striations represent tonal components at 1,350, 



.6 .8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 

TIME IN SECONDS 


Figure 10. Time-frequency-intensity analysis for sonic noise from a 5-knot cargo vessel. 


Figure 10 shows a record of the sounds pro- 
duced by a cargo vessel moving at 5 knots. 
This record was obtained by sampling the vari- 
ous frequencies very rapidly with a 45-cycle 
band-pass filter. The midfrequency of this 
filter increased almost linearly with time ; 
hence a vertical section of the record repre- 
sents the intensities of nearly simultaneous 
samples of narrow-band noise taken over a 
large range of frequencies. 

This record has three indicating scales and 
is one form of spectrum which shows time pat- 
tern. The scale of the vertical axis represents 


1,475, 1,725, and 1,850 cycles per second. The 
absence of propeller modulation from these 
tones indicates that they were probably pro- 
duced by the propulsion machinery. 

Figure 11 is a record of the noise produced 
by the same vessel moving at 10 knots; here 
the machinery tones are more nearly sub- 
merged by cavitation noise. Comparison of 
the two sets of traces shows that all the fre- 
quencies in cavitation noise are about equally 
affected by propeller modulation (see, however. 
Figure 12) and that the modulation rate in- 
creases with speed. 



0 .2 .4 .6 .8 UO 1.2 \A L6 1.8 2,0 2.2 2.4 

TIME IN SECONDS 


Figure 11. Time-frequency-intensity analysis for sonic noise from a 10-knot cargo vessel. 


the midfrequency of the filter; the horizontal, 
the passage of time; and the darkness of the 
trace, relative intensity. The relative dark- 
ness is adjusted, in these traces, to compensate 
for an intensity loss of 6 decibels per octave. 


^ Far from Source 

Sound may be changed in many ways during 
transmission through the sea."^ The following 

^ Many of the effects that occur are described in 
STR Division 6, Volume 8, Chapters 1-10. 


SOUNDS FROM SUBMARINES AND SURFACE VESSELS 


45 


summary of these changes is adequate for for example, is a record of changes in the level 
present purposes. The signal grows progres- of sounds received in a stationary hydrophone 
sively weaker with increasing distance from and produced by an approaching tanker, 
the source because of spreading and attenua- Stable interference patterns are considered 



TIME IN SECONDS 


Figure 12. Time-frequency-intensity analysis showing frequency modulation of the propeller 
sounds from a submerged submarine. The modulation occurs in the region of 0.5 kilocycle and coin- 
cides with the period of the propeller blades, rather than that of the shaft. The swish was a dis- 
tinctly audible component of the underwater sound. 


tion. The transmission loss is greater for the improbable at distances greater than 2,000 
higher frequencies; in other words, selective yards and at frequencies higher than 2 kilo- 
attenuation occurs. In any actual case, the re- cycles, since fixed phase relations between 


TIME IN MINUTES 

0 12 3 4 



Figure 13. Power level record of the signal from an approaching tanker. 


ceived signal is likely to show erratic fluctua- sounds traveling along two separate paths are 
tions produced by skip-distance effects, inter- not likely to persist beyond these limits. Such 
ference patterns, and other factors. Figure 13, effects may nevertheless be important in the 


46 


CHARACTERISTICS OF TARGET SOUNDS AND NOISE BACKGROUND 


case of a submarine taking evasive action. Un- 
stable interference effects, produced by con- 
tinual shifts in phase between sounds arriving 
by slightly different paths, seem to be respon- 
sible for the rapid variability of transmission 
loss which is observed at all distances and all 
frequencies. These rapid fluctuations in trans- 
mission may have adverse effects on the opera- 
tion of sound gear. The time scale of these 
fluctuations is generally different from that of 
propeller modulation and does not seem to hin- 
der the counting of propeller turns. 

A ship is an extended source; it is conse- 
quently possible to distinguish bow from stern, 
even at fairly long distances, by means of the 
change in quality of the sounds which are heard 
when the axis of a directional hydrophone is 
swept across the target bearing. 


3 2 INTERFERING BACKGROUND 

Many unwanted sounds enter the ears of 
sound operators. Since such sounds may mask 
faint signals, they set a range limit beyond 
which targets cannot be heard. The major 
sources of interference encountered in the lis- 
tening gear of surface vessels and submarines 
are described in the present section. 

Each of the sounds radiated by the listening 
vessel is a potential source of interference; 
hence the hydrophone should be shielded from 
them as carefully as possible. Since listening 
gear is tactically more effective when it can be 
used under way, it is difficult to attempt reduc- 
tion of interference by suspending the hydro- 
phone at a distance from the hull. Suspending 
cables tend to be unwieldy, and arrangements 
for steering the hydrophone to different bear- 
ings are not simple. 

In most installations used up to the present 
time, the hydrophone is mounted just outside 
the hull; its cables and suspensions are led 
through the hull and are shock-mounted to 
reduce vibration noise. The effectiveness of the 
hydrophone varies with its position on the hull. 
It should, for example, be well forward from 
the propellers and the engine room; it should 
be located where it will receive the least inter- 
ference from the auxiliaries and the bow wave ; 


and it should be placed where it will be shielded 
from the impact of bubbles carried by the slip- 
stream, but not where the hull will shield it 
from signals. On rapidly moving vessels, it 
has been found useful to enclose the hydro- 
phones in a streamline dome to reduce the noise 
arising from the flow of water around the 
hydrophone. 

The interfering sounds described so far are 
associated with the listening vessel and are 
generally termed self-noise. Self-noise in- 
creases with speed; it may be higher, for a 
given speed, when the vessel is in shallow 
water, as a result of reception of bottom-re- 
flected sounds. 

Interference may also arise from electrical 
noise, from airborne noise, and from ambient 
noise. Electrical noise refers to voltage varia- 
tions produced by stray electromagnetic fields 
or by random thermal fluctuations in the hydro- 
phone or its associated circuits. When these 
variations are amplified, they appear at the 
output as noise. Airborne noise does not origi- 
nate in the listening system. It reaches the 
operator from his immediate environment and 
may even be produced, in exposed locations, by 
the effect of air streaming past the headphones. 

Ambient noise consists of waterborne sounds 
which are not produced by the listening ship. 
It is a catch-all term applied to a large mixture 
of sounds. Some of the sources which contrib- 
ute to this mixture are ships other than the 
target, waves and surf, rain, marine life, ice- 
bergs, and underwater volcanoes. 

The level of ambient noise produced solely 
by motion of the sea surface (so-called deep-sea 
ambient) represents the lower limit of inter- 
fering background. Deep-sea ambient is essen- 
tially nondirectional ; the sounds come with 
equal strength from all bearings. The intensity 
of deep-sea ambient noise rises with increase 
of wind force and sea state. 


Sources 

Self-noise seems to be the dominant source 
of interference in sonic listening gear.® Am- 


® Sonic listening gear is designed to receive frequen- 
cies below 10 kilocycles; supersonic, to receive frequen- 
cies above 10 kilocycles. 


/ ^STRICTE^ 


INTERFERING BACKGROUND 


47 


bient noise is rarely a problem, except in heavy 
seas or when the listening vessel secures all its 
engines and auxiliaries. On the other hand, 
the level of self-noise received in the supersonic 
listening gear of slowly moving vessels is com- 
parable to that of deep-sea ambient noise, and, 
in high seas, supersonic ambient noise may ex- 


various frequencies in a hypothetical plane 
wave incident along the acoustic axis of the 
hydrophone; such a sound field, if it existed, 
would produce the same effect at the output 
as was actually observed. This is a convenient 
procedure because it represents all the noise 
sources on a common basis and in such a way 



FREQUENCY IN KC 

TOTAL SELF NOISE 

NOISE FROM PROPELLERS 

NOISE DUE TO MOTION THROUGH WATER 

-NOISE FROM ENGINES 

Figure 14. Components of sonic self-noise re- 
ceived in the listening gear of a small surface 
craft at 9 knots. Relative receiver bearing was 
000, sea state 1. 



TOTAL SELF NOISE 

NOISE FROM PROPELLERS 

NOISE DUE TO MOTION THROUGH WATER 

NOISE FROM ENGINES 

Figure 15. Components of sonic self-noise re- 
ceived in the listening gear of a small surface 
craft at 4 knots. Relative receiver bearing was 
000, sea state 1. 


ceed self-noise. It is usually possible to keep 
electrical and airborne interference to a fairly 
low level in listening gear on ships and sub- 
marines. 

The slopes of self-noise spectra are usually 
steeper than those of deep-sea ambient noise. 
This difference in slope tends to make self- 
noise relatively less prominent in supersonic 
gear than in sonic. Self-noise levels measured 
under way are highest when the hydrophone 
is pointed toward the propellers ; for other ori- 
entations, the levels are lower and approxi- 
mately independent of orientation. 

Figures 14 and 15 give some observed spec- 
tra showing the relative magnitudes of noise 
components which were present in sonic lis- 
tening gear installed on a small surface vessel. 
It is interesting to note the change in impor- 
tance of propeller noise at the two speeds 
shown and the tendency for total noise to 
approach ambient as the frequency increases. 
These spectra are plotted to show rms levels at 


as to simplify comparison with a signal spec- 
trum, but it is not intended to imply that such 
a noise field actually exists at the hydrophone. 
There is a distinction, therefore, between the 
sound-in-the- water (that is, at the hydrophone) 
and the sound-at-the-ear ; the sound meant will 
be specified in the remainder of this volume in 
all cases where the intended sense is not clear 
from the context. It is worth emphasizing that 
Figures 14 and 15 are presented for the sake 
of illustration only and should not be consid- 
ered typical of all sonic gear. 

Few background sounds have really distinc- 
tive character. Ability to detect apd classify 
elusive differences in background character is 
the mark of a competent operator, and it pays 
dividends. Noting changes in the sounds re- 
ceived from his own ship’s propellers and aux- 
iliaries is an important function of the sound 
operator on submarines, since such changes 
often mean increased detectability by enemy 
listening gear. This requires good memory and 


(^RESTRICTED 


48 


CHARACTERISTICS OF TARGET SOUNDS AND NOISE BACKGROUND 


good discrimination for intensity, pitch, qual- 
ity, and time pattern. Obviously, no operator 
can detect the slight change in background 
which is produced by a faint signal unless 
he is thoroughly familiar with the background ; 
conversely, an operator who consciously 
searches the background for features which 
resemble ordinary ship signals is less likely 
to report nonexistent signals. 

Some backgrounds contain more popping and 
crackling sounds than others. Such impactive 
sounds are present in all backgrounds to some 
extent but are almost universal in the ambient 
noise observed in tropical or subtropical shal- 
low-water areas over rock or coral bottoms. 
These sounds have been compared to the sound 
of frying fat; they are produced by colonies 
of snapping shrimp. Shrimp crackle is an im- 
portant source of noise because it is intense, 
incessant, and widely distributed. The spec- 
trum of shrimp noise shows a broad peak be- 
tween 10 and 30 kilocycles ; the level diminishes 
in the sonic range, and merges with deep-sea 
ambient in the neighborhood of 1 to 2 kilo- 
cycles. Ability to recognize shrimp crackle is 
important because it severely limits the ranges 
which can be obtained with supersonic gear; 
thus, shrimp noise may provide ideal acoustic 
camouflage for a slowly moving submarine. 


3 3 PROPERTIES OF LISTENING GEAR 

Studies of the performance of listening gear 
are described in Chapters 4 and 5. The fol- 
lowing summary of the properties and uses of 
listening gear provides a basis for evaluating 
the results of these listening studies. 

The chief function of listening gear is the 
detection and tracking of targets. Listening 
gear which can detect faint signals, in other 
words, which can detect targets at long range, 
is generally preferable to listening gear which 
cannot. On the average, longer detection ranges 
mean a greater number of contacts per cruise ; 
they also mean that more time and space are 
available for maneuvering. Within the range 
of validity of the inverse square law, the state- 
ment that “one listening system can detect a 
target at twice the range that another can” is 


equivalent to the statement “the flrst system 
can detect a signal which is 6 decibels weaker” ; 
in other words, 10 log (d/2d)^ = — 6 decibels, 
where d is the range obtainable with the second 
system. 

It is difficult to open or close the range ef- 
fectively or to compute torpedo courses unless 
the target bearing is known with adequate pre- 
cision. The bearing error which may be toler- 
ated with sound gear depends on the tactical 
situation and on the information obtainable 
from other types of equipment, such as radar 
or periscope. 

It has already been pointed out that an esti- 
mate of target bearing may be made by cross- 
ing the target with the axis of a directional 
hydrophone ; and that, at relatively short range, 
the bearings of bow and stern can often be 
determined. The qualitative change in the 
sounds which are heard during this transit 
permits the operator to judge the angular lim- 
its within which the target bearing seems to 
lie.^ The midpoint of the arc defined in this 
way is taken to represent the target bearing. 
The difference in degrees between the true 
bearing and the estimated bearing is the bear- 
ing error. The magnitude of the probable bear- 
ing error depends on the hydrophone directiv- 
ity and on various other factors (see Section 
5.1). 

The directivity of a hydrophone is deter- 
mined by the width of the cone of nonaxial 
sound rays for which the hydrophone response 
is of the same order of magnitude as its re- 
sponse to an axial ray (see Section 5.1). When 
a small, distant source is moved to various 
points on the surface of a sphere circumscribed 
about a stationary hydrophone and the hydro- 
phone response corresponding to various angu- 
lar positions of the source is represented on 
either a spherical or a polar plot, the resulting 
figure is called a directivity pattern. A similar 
figure is obtained when the source is stationary 
and the hydrophone is rotated about its own 
center. For most practical purposes it is suf- 
ficient to show a two-dimensional directivity 

* In other words, time pattern can be introduced in 
the received signal by crossing the bearing with a 
directional hydrophone. Thus, there are three types of 
time patterns: those due to the source, those resulting 
from transmission, and those produced by the operator. 




■: RESTRICXJgD 


PROPERTIES OF LISTENING GEAR 


49 


pattern representing the response in the hori- 
zontal plane, that is, the plane described by 
the acoustic axis of the hydrophone when the 
latter is rotated about its mechanical (vertical) 
axis. 


acoustic insulator (see Figure 2 in Chapter 5). 

Two aspects of these diagrams are worth 
noting. First, there are usually several sectors 
where the response is high; these are known 
as lobes. By proper design, the magnitudes of 




Figure 16. Theoretical directivity patterns for a 3-foot line hydrophone at 1, 2, and 5 kilocycles. The 
response is in decibels relative to axial response. 


Figures 16 and 17 show the horizontal di- 
rectivity patterns of two standard hydro- 
phones. The sensitive elements of these hydro- 
phones are assembled differently. In one case 


the secondary lobes can usually be made con- 
siderably smaller than that of the primary or 
main lobe. Thus, operators listening to a target 
at maximum range are unlikely to hear it on 




Figure 17. Theoretical directivity patterns for a 15-inch piston-type hydrophone at 5 and 25 kilo- 
cycles. The response is in decibels relative to axial response. 


(piston type), the receiving elements are ar- 
ranged on the surface of a circular disk; in 
the other (line hydrophone), along the surface 
of a narrow rectangle. In the case of the pis- 
ton, the mechanical axis coincides with a di- 
ameter ; in the case of the line hydrophone, with 
the minor axis of the rectangle. These figures 
show the front response of the hydrophones in 
decibels below the maximum response ; the rear 
response is diminished by use of a baffle, or 


a side lobe at one bearing, and again on the 
main lobe on another bearing;® the reason for 
this is that the background picked up on the 
main lobe will mask the relatively faint signal 
which can be picked up by orienting a side lobe 
toward the target. When the signal is well 
above background, it will be heard on the side 

g The bearing scale is usually calibrated in terms of 
angle between axis of the main lobe and some reference 
bearing, such as dead ahead or compass bearing. 






RESTRICTE 



50 


CHARACTERISTICS OF TARGET SOUNDS AND NOISE BACKGROUND 


lobes and may even give a reciprocal bearing 
when the limit of insulation afforded by the 
baffle is reached. When a loud target is picked 
up at several bearings, the operator can decide 
which is correct by comparing the relative 
loudness of the signals received at different 
bearings (the loudness difference is usually suf- 
ficiently great to make such a decision easy) or 
by reducing the gain so that signals received 
on the side lobes approach inaudibility. 

Secondly, it will be observed that the angular 
width of the main lobe varies inversely as the 
frequency of the sound for which the pattern 
is drawn. Thus, the available bearing accuracy 
will be greater at higher signal frequencies. If 
the perceived signal is distributed through a 
wide band of frequencies, the low-frequency 
components will show relatively little change 
of intensity when the hydrophone crosses the 
bearing, but there will be a comparatively large 
change in the intensity of the high-frequency 
components. In other words, the quality of 
the sounds heard will change when the target 
bearing is crossed ; the change of quality heard 
in this case is due to the difference between 
signal and signal-plus-background. Thus the 
operator can achieve greater bearing accuracy, 
when wide-band signals are heard, by focusing 
attention on the higher frequencies or by using 
high-pass filters. If a wide-band signal is weak, 
however, the use of filters may reduce it to 
inaudibility (see Section 4.1.6). When the sig- 
nal is strong, it may be dominated by its low- 
frequency components. Since these show a 
smaller intensity change when the target is 
crossed, and, therefore, give less bearing ac- 
curacy, it may be desirable to reduce the loud- 
ness of the low-frequency components of strong 
signals by dropping the amplifier gain. If the 
signal is sufficiently strong, reduced amplifica- 
tion also serves to eliminate false bearings 
picked up on the side lobes and to sharpen bear- 
ing accuracy by increasing the contrast be- 
tween the faint background which is heard 
off -bearing and the louder sound which is re- 
ceived on-bearing. 

When a directional hydrophone is placed in 
a nondirectional sound field, the hydrophone 
responds well to sounds incident on its axis 
and discriminates against nonaxial sounds. 


This reduction of background afforded by hy- 
drophone directivity improves performance by 
making it possible to detect weaker signals. 
The extent of discrimination against nondirec- 
tional noise increases with frequency. Figure 
18 gives the discrimination, against nondirec- 



Figure 18. Discrimination of hydrophones 
against nondirectional noise. 


tional noise at various frequencies, which is 
obtained with the hydrophones whose directiv- 
ity patterns are shown in Figures 16 and 17. 
The discrimination is plotted in decibels below 
the response which would be obtained at each 
frequency if an equally sensitive nondirectional 
hydrophone were placed in the same sound 
field. This number of decibels represents es- 
sentially the ratio between the solid angle de- 
fined by the prominent parts of the hydro- 
phone’s response pattern and the total solid 
angle about a point, An steradians. 

The total discrimination of a hydrophone 
against nondirectional sound is the sum of that 
shown in Figure 18 and that provided by the 
baffle. The latter suppresses approximately 
half of the response pattern (the rear re- 
sponse) for most hydrophones at most fre- 
quencies; less suppression is provided at low 
frequencies, because of diffraction, and near 
the resonance frequency of the baffle. On the 
average, the baffle adds about 3 decibels of 
discrimination. 

Hydrophone directivity improves with in- 
creasing frequency and with increasing hydro- 
phone dimensions. Since there is an upper 
limit to the size of practical hydrophone instal- 
lations, supersonic listening gear usually gives 
better bearing accuracy and better discrimina- 
tion against nondirectional background than 


(| ^^STR1CTE|^ 


PROPERTIES OF LISTENING GEAR 


51 


can be obtained with sonic listening gear. 
Large sonic hydrophones have been constructed 
by connecting a group of small units in series. 
Figure 24 in Chapter 4, for example, indicates 
the discrimination against nondirectional back- 
ground which is obtained from an assembly of 
48 hydrophones arranged in the form of a ring 
8 feet in diameter. The acoustic axis of such 
an assembly can be oriented by introducing 
various amounts of electrical delays in the lines 
connecting the individual units. Thus, if the 
output of units closer to the source are delayed 
an appropriate amount, they are brought into 
phase with the output of units farther from 
the source. In this condition, the total output 
of the assembly is a maximum ; in other words, 
the hydrophone is on-bearing. Hence, target 
bearing may be determined from the amount 
of delay needed in the various lines. 

Figures 19 and 20 illustrate the mechanism 
of hydrophone directivity for an idealized case. 
Here, XW represents a progressive plane wave, 
of wavelength A, incident on a hydrophone face 
YZ. The wave moves in the direction OM, M 
being the midpoint of the face, and d its length ; 
thus, the angle of incidence is 6. When 9 is 
zero, all the parts of the surface YZ move in 
phase, and the hydrophone output is maximal. 
As 9 increases, a situation like that shown in 
the figure occurs ; in other words, elements such 



Figure 19. Mechanism of hydrophone directivity. 

as e and e\ selected from the two halves of the 
hydrophone and symmetrically placed with re- 
spect to its midpoint, are out of phase by 180 
degrees. Under these circumstances, the re- 
sultant output represents the net effect of ele- 
ments which cannot be paired in this way. 


When the effect of all the elements on one 
half face completely cancels the effect of the 
elements on the other half face, the output 
falls to zero. As shown in the figure, this oc- 
curs when sin 9 equals X/d, that is, when the 


A 


N 




+-H 




tY 


Figure 20. 
ure 19. 


Phase of disturbance shown in Fig- 


hydrophone face intercepts all the parts of the 
disturbance corresponding to a single wave- 
length. As 9 increases beyond the value shown 
in the figure, the symmetry existing between 
the two halves of the hydrophone diminishes; 
hence cancellation is incomplete and the output 
is increased. The output then reaches another 
relative maximum because of increased rein- 
forcement among the various elements and then 
falls to a second minimum when 9 assumes such 
a value that the hydrophone intercepts exactly 
two wavelengths, that is, when sin 9 = 2x/d. In 
other words, there are a group of response 
lobes, the minima occurring whenever an in- 
tegral number of wavelengths is intercepted by 
the hydrophone, that is, minima occur for all 
values of 9, in the expression 9 = arc sin 
(nx/d), which correspond to integral values 
of n. 

The width of the main lobe is simply the 
angular distance between the first minimum on 
one side of the axis and the corresponding first 
minimum on the other side. Since the first 
minimum falls at an angle 9 such that sin 
9 = X/d, the width of the main lobe is 2 arc sin 
{x/d). When A is considerably smaller than d, 
the angle 9 is equal to its sine, and thus the 
width of the main lobe is 2x/d radians. If c/f 
is substituted for A, where c is the velocity and 
/ the frequency of the sound incident on the 
hydrophone, the lobe width equals 2c/ fd radi- 
ans. In other words, the lobe width, and hence 
the response to nonaxial sounds, decreases 
when either the dimensions of the hydrophone 
or the incident frequency is increased. It is 
evident that when A exceeds d, the phase differ- 
ences across the hydrophone are negligibly 
small, and the hydrophone loses almost all di- 
rectionality. 


^RESTRICTED ^ 


52 


CHARACTERISTICS OF TARGET SOUNDS AND NOISE BACKGROUND 


The magnitude of the response along the 
axes of the side lobes is smaller than that 
along the axis of the main lobe (0 = 0 degrees) 
because some cancellation always occurs be- 
tween the two halves of the hydrophone face 
when 0 exceeds zero. Thus, response patterns 
for a given hydrophone show an increasing 
number of side lobes for higher frequencies, 
but all the lobes are narrower for the higher 
frequencies, and the response on the side lobes 
is usually very small compared with that ob- 
tained on the main lobe. A directional hydro- 
phone will therefore pick up a smaller fraction 
of the high-frequency energy in a nondirec- 
tional sound field than of the low-frequency 
energy ; in other words, it has a greater degree 
of discrimination against high - frequency 
sounds. 

3.4 TYPES OF MASKING 

The preceding discussion has been devoted 
to the separate characteristics of signal and 
noise spectra as received by listening gear. 
Before discussing the observed masking data, 
it may be helpful to survey the problem broadly 
and to examine various types of masking situa- 
tions that may arise. First, however, the con- 
cept of primaudibility, which is basic in the 
study of masking, is defined. 

Definition of Primaudibility 

In order to distinguish between threshold- 
limited listening (audibility determined by 
sound level relative to the absolute ^audibility 
threshold) Bjod masking -limited listening (audi- 
bility determined by level of signal relative to 
a background which itself lies above the audi- 
bility threshold), it has been suggested that the 
coined word primaudible (from prime plus 
audible) be used to describe the faintest signal 
which is audible under masking-limited condi- 
tions. This general term should be qualified 
to denote the percent detection probability 
which is intended (such as “50 per cent 
primaudible”) ; where it is not qualified in the 
present discussion, a detection probability of 
50 per cent is understood. The term audible is 
thus reserved for threshold-limited listening. 


Since detectors other than the ear may be 
used with sonar equipment, it has been pro- 
posed that the word priceptible (from prime 
plus perceptible) be employed as a general term 
to describe the faintest signal which can be 
perceived by the detector under masking-lim- 
ited conditions, and that the term be qualified 
as above with respect to detection probability. 
Since these definitions make for brevity and 
precision, their use has been adopted in this 
volume. 


^ Masking by Single Tone 

In this case, the masking curves of Figure 2 
in Chapter 2 apply. When the signal is a pure 
tone whose frequency approximates that of the 
masking tone or its subjective harmonics, the 
cue to recognition is essentially a fluctuation in 
loudness resulting from beats between the two 
tones. The minimum perceptible intensity in- 
crement determines the ratio of signal to noise 
at primaudibility. 

For a pure tone of other signal frequencies, 
the limiting factor is competitive stimulation 
of the basilar membrane by the masking tone 
(remote masking). The amount of this stimu- 
lation can be determined only by empirical 
methods at the present time, but a number of 
observed regularities have been listed in Sec- 
tion 2.1.2. The effects of frequency and ampli- 
tude modulation have not been studied for the 
present case. 

When the signal has a continuous spectrum, 
the criterion for audibility is presumably that 
the signal intensity in at least one of the ear’s 
critical bands be equal to or greater than the 
intensity of the single tone which would be 
primaudible at that frequency. No detailed 
laboratory evidence is available, however, to 
test this expectation. 


3.4.3 jviaskiiig by Continuous Background 

If the background has a continuous spec- 
trum, the limiting factor is competitive stimu- 
lation of the basilar membrane, and the criti- 
cal band criterion applies. Recognition should 
then be possible when the signal energy in at 


RESTRICTED 


TYPES OF MASKING 


53 


least one critical band is equal to the back- 
ground noise energy in the same band. When 
the isolated signal components are widely dif- 
ferent in pitch and character, the critical band 
criterion applies to each component individu- 
ally. In other words, the various signal com- 
ponents do not necessarily cooperate to improve 
signal audibility; in fact, it is possible to hear 
one component and miss the other, or others 
(see Sections 4.1.3 and 4.1.5). When the sig- 
nal components lie within a narrow frequency 
interval, the group may or may not be more 
audible than the individual components. 

When signal and background spectra have 
similar compositions within a frequency inter- 
val extending over many adjacent critical 
bands, the cue is usually change in loudness, 
and the condition which determines primaudi- 


bility is the size of the intensity limen (least 
perceptible intensity increment). 

3 . 4.4 Masking by Group of Tones 

The case in which the masking background, 
as well as the signal, consists of a group of 
tones has not been studied. It will be obvious, 
however, that all the masking situations al- 
ready discussed above are limiting cases of this 
last situation and may be approximated by 
increasing or decreasing the number and the 
frequency separation of the tones in the signal 
and background. It is possible, therefore, that 
useful estimates of the masking of a group of 
tones by another group may be obtained by 
interpolating among observations made on the 
preceding simpler types. 


Chapter 4 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


I ISTENING GEAR provides a means for achiev- 
ing certain practical objectives. The extent 
to which current gear may be expected to meet 
the objectives set for it and the likeliest meth- 
ods for getting better results depend in part on 
the ability of the human ear to detect target 
sounds. While the basic information on the 
structure and performance of the ear, described 
in Chapters 1 and 2, helps to answer many 
questions, the extent of the ear’s ability to de- 
tect underwater sounds generated by surface 
vessels, submarines, torpedoes, and other 
sources can best be decided by experimental 
test and analysis. This program has made con- 
siderable progress but is, in many ways, still 
incomplete. Chapters 4 and 5 set forth its 
major findings and indicate briefly some of the 
things which remain to be done. 

The purpose of the listening tests described 
in these chapters is to determine how well the 
ear of an average, competent operator can per- 
form certain assigned tasks. The general na- 
ture of these tasks and the broad features of 
the hearing process have been outlined in the 
preceding discussion. 

Two general types of test have been con- 
ducted: (1) laboratory tests, in which the field 
situation is simulated as faithfully as seems 
necessary and practicable, and (2) field tests, 
in which the listener is stationed aboard ship 
and the listening situation closely approximates 
the one encountered in practice. The laboratory 
measurements, made under controlled condi- 
tions, are described in this chapter, while the 
field measurements are discussed in the follow- 
ing chapter. Both types of test provide useful 
information, and both types have character- 
istic merits and defects. Field tests, for ex- 
ample, can be made extremely realistic, but 
they are time-consuming ; also they require that 
personnel, marine facilities, and favorable 
weather be available simultaneously. Further- 
more, field tests yield rather uncertain conclu- 
sions unless (1) the factors which determine 


performance are accurately measured and con- 
trolled, or (2) a statistically significant series 
of tests is made. These requirements are diffi- 
cult to meet in the case of field tests. They are 
readily met in laboratory tests, but such tests 
are inevitably artificial. Hence considerable 
care is necessary in the design and interpreta- 
tion of laboratory tests if practically useful in- 
formation is to be obtained from them. 

Listening tests are intended to provide defi- 
nite, quantitative information. In other words, 
such tests are essentially measurements; like 
other measurements, they must be properly 
planned and performed if they are to be help- 
ful rather than misleading. Thus, the sounds 
used in the tests should be typical of those met 
in practice. The observers should have normal 
hearing, and, if possible, the same or compar- 
able observers should be used throughout a test 
series. The observers should understand the 
situation and know exactly what they are ex- 
pected to do. When the test situation is new 
to the observers, their performance usually im- 
proves during the time needed to learn the 
ropes. It is desirable to make provision for 
such preliminary adjustment in order to ap- 
proximate the performance to be expected from 
trained personnel. Even after such preliminary 
adjustment, the test observers may not do as 
well at the assigned task as would experienced 
field personnel. It seems reasonable to expect, 
however, that any advantage in favor of per- 
sonnel accustomed to a particular routine would 
be offset by various factors, such as fatigue, 
boredom, and distractions, which may affect 
performance under service conditions but 
which do not usually enter the test situation. 
Finally, precautions must be taken to eliminate 
guessing and practice effects, such as memoriz- 
ing the test items or anticipating the order in 
which they are presented. 

The preceding requirements apply to all test 
programs; they also apply to programs de- 
signed to train personnel in the effective use of 


^ RESTRICTE^ 


54 


BRITISH TESTS 


55 


sound gear. Other points of procedure will 
emerge in the following discussion of observa- 
tions. These observations are described in semi- 
chronological order, and the discussion is inter- 
spersed occasionally with general material not 
contained in the original reports. This method 
of presentation has been adopted because of 
dissimilarities in the techniques and materials 
of observation which were used at different 
laboratories. Although most of these dissimi- 
larities are of minor significance, it would bur- 
den the discussion to keep all the distinctions 
to the fore at all times. Specific details of ap- 
paratus and test administration and analysis 
are generally omitted. 


BRITISH TESTS 

This group of tests'-^ was a pioneering study 
undertaken to examine the factors affecting 
aural detection of sonic submarine sounds 
masked by deep-sea ambient noise. Among the 
factors studied were the choice of observers, 
method of presenting the sounds used, gain 
setting, filters, and discrimination of directional 
gear against nondirectional background. The 
apparatus used is shown schematically in Fig- 
ure 1. Recordings of the signal and the back- 
ground, suitably amplified, were fed to a mix- 
ing panel and, after further amplification, the 
mixture of sounds was presented to the observ- 
ers through headphones. In a given series of 
tests the setting of the background amplifier, 
as well as that of the mixture amplifier, was 
maintained constant, and the setting of the 
signal amplifier varied in steps of 2 decibels. 
The observers were seated in a quiet room. A 
selected mixture of sounds was presented to 
them for 15 seconds, and, in the silent interval 
before the next mixture was presented, they 
recorded whether the signal was “heard’' or 
“not heard.” This procedure determined the 
signal-to-noise ratio needed for detection. The 
required measurements of signal and noise 
levels were made with the calibrated measuring 
amplifier shown at the right in Figure 1, and 
the results obtained are presented in Figures 6 
through 23. The information accompanying 
these figures describes the source of each sound 


and its characteristics as heard in the absence 
of the masking background. The chief feature 
of interest in these figures is the relation exist- 
ing between the spectrum of the masking back- 
ground and the spectrum of the signal which 
can barely be detected. In general, they indi- 


SOUND HEAD 1 SOUND HEAD 2 




Figure 1. Schematic diagram of listening test 
apparatus. 

cate that recognition of a primaudible wide- 
band signal tends to occur at the frequency 
where the signal spectrum level is highest rela- 
tive to background. Other significant aspects 
of the observations are discussed in the follow- 
ing sections. It should be noted, however, 
that a number of the terms and concepts used 
in the discussion of this group of tests were not 
explicitly stated prior to publication of the 
work described in Section 4.2. 


AMPLI- 

RER 


MEASURING 

AMPURER 


6 


RESTRICTED 


ir? 


56 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


4.1.1 Measurement of Sounds 

The noises used were obtained from film 
recordings of submarine and water sounds. 
These recordings were made at a sound range 
and were obtained with a system whose overall 
response was quite fiat from 60 to 8,000 cycles. 
The recording hydrophones were close to the 
submarine, so that all but the faintest subma- 
rine sounds were well above background noise 
present in the water. Representative sections 
of the films were selected, the ends of the sec- 
tions spliced to form continuous loops, and the 
recorded sounds were reproduced by running 
the loops on one of the sound heads shown in 
Figure 1. The advantage of film over disk 
recordings, for work of this sort, is that film 
recordings are less susceptible to changes pro- 
duced by wear. The use of recordings for listen- 
ing tests minimizes the amount of time during 
which the target vessel is unavailable for other 
assignments. 

Two different sets of recordings were used 
for these tests; those involved in Figures 6 
through 15 were made at an earlier date than 
the ones described in Figures 16 through 23. 
The recordings of signal and background which 
were used in these listening tests were always 
obtained with the same recording system. Fail- 
ure to observe this precaution may give quite 
unrealistic results when the system response is 
very different for signal and background record- 
ings ; for example, false peaks and hollows may 
be introduced into the spectra. 

The use of an ambient-noise background for 
these tests corresponds to one tactical situation 
of interest, since antisubmarine vessels, with 
the aid of a cable-suspended hydrophone, some- 
times conduct a search while adrift. Knowledge 
of the level at which a given submarine sound 
can be heard in the presence of water-noise 
background may then be interpreted in terms 
of the range within which a submarine can be 
detected by listening. This requires adjust- 
ment for the energy lost in transmission of the 
sound from submarine to searching vessel. Due 
allowance must therefore be made for such 
factors as selective attenuation and interfer- 
ence patterns produced by direct and surface 
reflected sound (see Section 3.1.2). Such range 


information is equally useful in prosubmarine 
and antisubmarine work. Roughly, a required 
increase of 6 decibels in signal level means the 
detection range is halved. 

Several sources of error must be considered 
when the results of laboratory listening tests 
are used in detection-range calculations; these 
factors must also be considered, therefore, 
when the laboratory tests are planned. The sig- 
nal used may not be typical of the class. It may 
be subjected to amplitude and frequency dis- 
tortion if nonlinear systems are used in the 
recording or presentation of sounds; in par- 
ticular, the mixture amplifier in Figure 1 may 
be troublesome in this respect, unless properly 
designed. Weak signals may contain large 
amounts of background noise. Thus, the high- 
frequency^ output of the signal records used in 
these tests contained appreciable ambient noise, 
except for the cases shown in Figures 11, 12, 
14, 16, and 17. This situation is artificial, since 
it may lead to “detection’’ through increased 
loudness of the recorded ambient noise. 

One of the fundamental purposes which a 
program of listening tests can serve is to pro- 
vide a method for predicting the audibility of 
signals on the basis of spectrum, time pattern, 
and other characteristics. The spectra of signal 
and background used in these tests were de- 
termined by measuring the fraction of the 
acoustic energy in the sounds which was trans- 
mitted by each of a set of octave band-pass 
filters. Spectrum level of the noise background 
is plotted in decibels below the overall energy, 
that is, the energy contained in all the frequen- 
cies between 50 and 10,000 cycles per second. 

Octave filters are designed to transmit only 
the energy contained between the frequency 
limits / and 2/, 2/ and 4/, 4/ and 8/, and so on 
To find the average energy per cycle, which is 
called the spectrum level when expressed in 
decibels, the ratio //A/ is formed, where / is 
the energy passed by the filter and A/ is the 
number of cycles between the upper and lower 
frequency limits of its pass band. 

a For practical convenience in description, the follow- 
ing approximate sonic frequency intervals are distin- 
guished: low frequencies are those below 2 kilocycles; 
middle frequencies include those between 2 and 5 kilo- 
cycles; and high frequencies extend from 5 to 10 
kilocycles. 




BRITISH TESTS 


57 


The relations between the arithmetic mid- 
frequency F of an octave filter and the lower 
and upper cutoff frequencies /i and of that 
filter are shown in Figure 2. Since the pass 

^ Af ► 


Af /2 ► 


J_ 

F 


f2 = 2f, 


Figure 2. Characteristic of an octave filter. 


2 2 2 2 

(1) 

hence 


A/=/i. 

(2) 

If F is the arithmetic midfrequency, then 


. A/_3 

^ Tyfi, 

(3) 

and 


/i = W. 

(4) 

Then from (2) and (4), 


Af = IF; 

(5) 


in other words, the number of cycles in the pass band of an 
octave filter is equal to f of its arithmetic mid-frequency. 


band A/ of such a filter includes an increasingly 
greater number of cycles as the midfrequency F 
is raised, the use of octave filters shows pro- 
gressively less detail for the higher frequencies 
in the spectrum. In fact, the pass bands of 
octave filters exceed critical band widths for all 
frequencies above 100 cycles (see Figure 17 in 
Chapter 2) ; octave filters are therefore poorly 
adapted to studies in which the ear’s critical 
bands play a part. 

When measurements made with octave filters 
are reduced to a “per cycle” basis, it is neces- 
sary to select a frequency, within the pass band 
of the filter, to which the computed value of 
the average energy per cycle is to be assigned. 
If the spectrum is fiat, that is, if the energy 
per cycle is the same for all frequencies, it is 
sufficient to select the arithmetic midfrequency 
of the filter. In most noise spectra, however, 
the energy is concentrated in the lower fre- 
quencies; the common practice, therefore, is to 


assign the deduced average energy value to the 
geometric midfrequency yjjo of the octave 
filter. As shown in Figure 3, this procedure is 
valid only for spectra which have a slope of 
— 6 decibels per octave (when I = k/f^, where k 
is a constant), but it is a fair approximation 
for other slopes as well. In obtaining the spec- 
tra shown in Figures 6 through 23, it has been 
assumed that (1) the filters used were “rectan- 
gular” (in other words, that they had sharp 
cutoffs), (2) the average energy per cycle 
should be assigned to the geometric midfre- 
quency of the filter, and (3) the spectra con- 
tained no tones or other discontinuities. These 



Figure 3. Effective midfrequency of an octave 
filter. The problem is to find the frequency 
where the actual value of I equals the average 
value I. Thus, if the area under the curve equals 
the area of the rectangle. 




/2 




Therefore 


or 


_k k_J.-Sr 

/l /2 /l/2 



( 1 ) 


fx = '\//i /2 = = 1.414/i (3) 

1.414(fF) = 0.94F. 


assumptions are probably close to the truth for 
the case of the ambient background, but the 
last assumption is almost certainly untrue for 
all the signals, except cavitation noise (or 
“propeller thrash”). This is indicated by the 
recurrence of the words “hum” and “tone” in 
the descriptions of the signals. The energy con- 
tent of these tones is unknown, but they must 
have been some 15 decibels above the adjacent 




58 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


spectrum level of the signal in order to escape 
masking (see Figure 17 in Chapter 2) . 

It will be noted, in Figures 6 through 23, 
that there is invariably a flat peak in the spec- 
tra in the frequency region where tones were 
detected. Such peaks would be expected if 
spectra contained tones superimposed on dis- 
tributed sounds. Obviously, the effectiveness of 



4681 Z 4681 2 4681 

lOO 1000 10,000 

FREQUENCY IN CYCLES 

Figure 4. Fluctuations in water noise produced 
by occasional loud “plonks” due to waves lapping 
the side of the recording vessel. The mean level 
of water noise produces a loudness of 70 phons. 

such a tonal peak in raising the total energy 
measured through a wide-band Alter dimin- 
ishes as the filter width increases. 

Most of the energy measurements were made 
with a sluggish instrument, a copper oxide 
meter, and it should be noted that rhythmic or 
random peaks in sound level are not indicated 
very well by such a device. The peak factor 
(which refers, in the present discussion, to the 
difference in decibels between mean and peak 
values of rms level) for machinery sounds of 
the type shown, as estimated with the copper 
oxide meter, was found to be about 3 to 5 deci- 
bels. A similar peak factor applies to the back- 
ground noise. Thus in Figure 4, the upper 
dashed curve represents occasional peaks in the 
background noise which were produced by 
waves lapping the side of the recording vessel. 
The spectrum levels corresponding to these 
“plonks” were obtained with a power level 
recorder. The continuous line in this figure 
shows the mean level of background noise dur- 
ing the intervals between wave slaps ; the 
dashed lines show the upper and lower limits of 


deviations from this mean level. The dot-dash 
curve is discussed later. 

The background spectra shown in Figures 6 
through 15 represent the composition of the 
sound between wave slaps. The pitch of this 
sound changed somewhat during the course of 
the record. It is therefore represented in Fig- 
ures 6 through 15 by two analyses, taken at the 
beginning and the end of the record, respec- 
tively, and the mean trend of these analyses is 
given by the continuous curve in Figure 4. 
Marked fluctuations in the levels of signal 
spectra are shown in Figures 11 and 15. It is 
interesting to note that the background spec- 
trum given in Figures 16 through 23 does not 
exhibit such marked variability, presumably 
because the water noise shown was recorded 
with a hydrophone mounted on the bottom 
instead of near the surface. 

The trend of the nondirectional background 
spectra shown in these figures is —6 decibels 
per octave. The spectrum level of the nondirec- 
tional background at 300 cycles is about 30 
decibels below the overall level of the back- 
ground. When applied to sonic spectra, the 
term overall level means the energy in the 
band from 0.1 to 10 kilocycles; this band of 
frequencies is termed the standard reference 
hand. The frequency limits of the spectra 
shown in these figures very nearly coincide 
with those of the standard reference band. As 
indicated later, the limits of the standard refer- 
ence band are imposed by the responsiveness of 
sonic listening devices and the sensitivity of 
the ear. 

This relation between the overall level and 
the spectrum level at 300 cycles may be verified 
by using the method indicated in Figure 3, that 
is, by evaluating the energy in the standard 
reference band for various spectra and deter- 
mining the fractional part of that energy in a 
1-cycle band at 300 cycles. The relation is often 
useful in working with spectrum levels, and is 
a good approximation for all distributed spectra 
with constant slopes of —3 to —9 decibels per 
octave. 

In contrast with the background spectra, the 
energy distribution of the machinery sounds 
shows no uniform dependence on frequency 
other than that it tends to peak at the low fre- 



BRITISH TESTS 


59 


quencies. In other words, machinery spectra 
tend to be individualized. It is interesting to 
compare the spectra of the sounds produced 
when the bow and stern planes are operated 
(Figures 6 and 8). Similarly, Figures 7 and 21 
show the difference in output of ballast pumps 
on two submarines. The spectra of the sounds 
shown in Figures 13A and 13B are not so much 
typical of speech alone as they are of speech 
coming over a loudspeaker and transmitted 
through a submarine hull. Figures 14 and 15 
indicate the character of the noise radiated at 
or below the cavitation threshold; Figures 16 
and 17 show well developed cavitation, and Fig- 
ure 18 illustrates an intermediate stage (see 
also Figures 4 and 5 in Chapter 3). Finally, 
Figures 19 and 20 show the increase in promi- 
nence and frequency of gear whine produced by 
increasing speed, and Figures 22 and 23 show 
a similar effect for pump noise. 

The spectra reproduced in Figures 6 through 
23 give the distribution of energy in the sounds 
used, as measured at the output of the mixture 
amplifier shown in Figure 1. This distribution 
is not identical with that occurring at the out- 
put of the headphones. In other words, the 
sound at the ear may be significantly different 
from the sound in the water.*" 

The factors which affect the performance of 
headphones, in addition to their own electrical 
and mechanical characteristics, are (1) the 
volume of enclosed air, (2) leakage around the 
cap, which is probably most significant for fre- 
quencies lower than 500 cycles, (3) the con- 
struction and resonances of the ear canal, (4) 
the yielding of the drum membrane and the 
walls of the canal, and (5) the directional char- 
acter of the phones. In other words, the per- 
formance of headphones depends upon the im- 
pedance into which they work. 

It should also be mentioned at this point that 
temperature changes encountered in practice 
may significantly alter the performance of 
headphones. Similarly, slight changes in the 
position of the phones with respect to the ears 
may produce large changes in the loudness of 
their low-frequency and high-frequency out- 
puts. 

^ Failure to take this factor into account seriously 
weakens several of the conclusions in reference 28. 


^ ^ Estimated Audibility Threshold 

Figures 6 through 23 show the relation exist- 
ing between the spectra of signal and back- 
ground when (1) the signal-to-noise ratio was 
sufficient to give detection in half the presenta- 
tions, and (2) the loudness level of the received 
sound was observed to be approximately 70 
phons, in other words, the intensity of the 
equally loud 1,000-cycle tone was 70 decibels 
above threshold. This is a comfortable loudness 



4661 2 4661 2 4661 

100 1000 10,000 


FREQUENCY IN CYCLES 


Figure 5. Estimated sensation levels of water 
noise, relative to the absolute threshold for 
tones. Solid line shows critical-band levels of 
average water noise relative to overall noise 
level. Dashed lines indicate positions of thresh- 
olds for pure tones when the presented water 
noise has the loudness levels shown on the 
curves. 


for continuous listening. It is important to 
know the level above threshold for each fre- 
quency under the conditions of these tests. 
While this cannot be determined accurately for 
these British data, the analysis may be instruc- 
tive. 

The level of background relative to the abso- 
lute audibility threshold of an average observer 
taking these tests may be estimated by the 
methods mentioned in Section 2.3, in other 
words, by finding the fractional part of the 
total energy (in the mean water noise shown 
by the unbroken line in Figure 4) which is con- 
tained within the limits of each critical band 
stimulated by the presented sound, and sum- 
ming the loudness contributions made by all the 
stimulated bands. The solid line in Figure 5 


RESTRICT] 


60 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


represents computed energy levels in critical 
bands at various frequencies and differs in 
shape from the 1-cycle spectrum of the back- 
ground shown in Figure 4 because critical 
band width is a function of frequency (see Fig- 
ure 17 in Chapter 2). The dashed curves in 
Figure 5 give the headphone threshold for pure 
tones (see Figure 1 in Chapter 2). To conform 
to the practice followed by the British, all 
curves in this section are plotted in decibels 
relative to the overall background. Thus, the 
relative position of the pure-tone threshold 
plotted on this basis will depend on the overall 
level; hence three threshold curves have been 
drawn to correspond to overall levels of 50, 70, 
and 90 phons. 

The spectra in Figure 4 are represented on 
the basis of energy per cycle Hence, the 
energy /c& available for stimulating a critical 
band of width Afch is Ici, = Afcb, or 10 log 

= 10 log Icb — 10 log Afch. As pointed out 
above, this is the relation connecting the aver- 
age water-noise spectra shown in Figures 4 and 
5. To a good approximation, the same expres- 
sion describes the relation between the tonal 
threshold It at a frequency /, and the threshold 
intensity Id of a distributed sound extending 
between the limits of the critical band centered 
at /; in other words, 10 log Id = 10 log It — 10 
log Afch, where Id expresses the energy per cycle 
when a distributed sound attains its threshold 
value. The quantity computed from the last ex- 
pression may be termed the “threshold for 
distributed sounds.” In Figures 6 through 23, 
which are plotted on a per cycle basis, this 
threshold for distributed sounds is shown for 
an estimated background loudness level of 70 
phons. 

It should be noted at this point that the 
spectra and the threshold shown in Figure 5 
are to some degree inconsistent, since the com- 
position of the water noise was measured at the 
input to the headphones whereas the illus- 
trated tonal threshold refers to their output. In 
the absence of information on the properties of 
the headphones used in these tests, there is no 
quantitative basis for revision of Figure 5. If 
the response of the headphones used was not 
approximately flat over a large part of the 


sonic frequency range, the curves plotted in 
Figures 4 and 5 may have little relevance for 
these British tests. 

In general, however, the illustrated relations 
are in qualitative accord with observations 
made in these and other tests. For example, the 
relation between the critical-band spectrum of 
water noise presented at 70 phons and the tonal 
threshold, shown in Figure 5, is in fair agree- 
ment with the data shown in Figure 76, which 
represents a similar situation, except that the 
measurements used in plotting both the thresh- 
old and the spectrum shown in Figure 76 were 
made at the input to the headphones. Thus, the 
estimates furnish a useful picture of the ap- 
proximate conditions of test and emphasize one 
type of measurement which is required to sup- 
plement the straightforward masking studies. 

Although audibility thresholds and head- 
phone outputs have usually been ignored in 
listening tests, it is clear that these quantities 
may signiflcantly affect the nature and inter- 
pretation of results, as well as the recommended 
design requirements. In the case at hand, it ap- 
pears doubtful, on the basis of the estimated 
audibility threshold and the probable head- 
phone response, whether much weight should 
be given to the spectra in the regions below 100 
or above 7,000 cycles. Between these limits, 
however, the slope of the background spectrum 
seems well adapted to optimal listening; the 
level of the background noise above threshold 
does not change as much in this frequency 
range as would be the case for a flat back- 
ground. Consequently, the background plotted 
in Figure 5 tends to produce the sensation of a 
“full” or “well-balanced” sound since it gives 
more nearly equal emphasis to all the frequen- 
cies than a flat background would give.*" To ob- 
tain a completely “well-balanced” sound, the 
noise level in each critical band should produce 
the same subjective loudness. Thus, the de- 
crease of the background relative to threshold 

Such a sound is often called a “white” noise, but 
this term is also used to describe a continuous spectrum 
with the same amplitude at all frequencies. Since it is 
not always clear from the context whether the expres- 
sion is intended to mean “objectively” or “subjectively” 
white, spectra are usually described in the remainder 
of the discussion by means of their slopes, in decibels 
per octave. 


SXRl CT^^j 


BRITISH TESTS 


61 


shown for low frequencies in Figure 5 is proba- 
bly desirable in order to give these frequencies 
equal loudness with the others. 

The advantage of a well-balanced sound is 
presumably decreased operator fatigue. With 
such a sound it is equally easy for the operator 
to focus attention on each portion of the fre- 
quency band, and the high-frequency compo- 
nents are not annoyingly loud. The masking 
of sounds will not be directly affected by the 
slope of the background, unless this slope is 
so steep that remote masking occurs (noise in 
one critical band masks a sound in another). 
For most backgrounds, the noise frequencies 
masking the signal are only those falling with- 
in the same critical band. Thus the slope of 
the background, while it has some importance 
in operator fatigue and overall effectiveness, 
is not of crucial importance in the detection 
of target sounds. 

The validity of this conclusion and of the 
critical-band criterion for masking of distrib- 
uted sounds depends, of course, on the impor- 
tance of remote masking. While little quantita- 
tive information on remote masking for dis- 
tributed sounds is available, it seems probable, 
on the basis of Figure 6 in Chapter 2 and 
Figures 76 through 79, that this problem will 
not arise if the 1-cycle spectrum of the pre- 
sented sound has a slope not exceeding some 
25 decibels per octave, or in terms of Figures 
6 through 23, if the slope of the presented 
sound is not significantly steeper than the slope 
of the audibility threshold for distributed 
sounds. Under such circumstances, the critical- 
band criterion for masking of tones by distrib- 
uted sounds should apply without modification ; 
this conclusion agrees with the results dis- 
cussed in Section 4.2.5. 

If the response of the headphones were flat 
and the loudness level were known to be accu- 
rately 70 phons, the signals plotted in Figures 6 
through 23 would be audible only if they lay 
above the plotted thresholds. However, the 
overall background may have differed by as 
much as 5 decibels from the estimated 70 phons. 
In addition, the headphones may not have had 
a flat response, especially at the frequencies 
below 500 cycles, at which the signal tends to 


lie close to or below the plotted threshold 
curves. For this reason, no great reliance can 
be placed on these computed thresholds. They 
are included here primarily to indicate the 
general possibility of analyses along these lines, 
rather than to draw precise conclusions from 
the data. 


Observed Recognition Differentials 

The relation which existed between the sig- 
nal and background spectra, when the signal 
was just recognizable, was determined by plot- 
ting a transition curve and adopting 50 per 
cent recognition as the criterion of detectabil- 
ity. The term “transition curve” refers to a 
plot of percentage recognition against signal- 
to-background ratio, where percentage recog- 
nition is defined as the fraction of trials at a 
given signal-to-background ratio for which the 
signal was heard. The signal and background 
energies are measured in the standard refer- 
ence band, and their ratio is usually expressed 
in decibels. The transition curves obtained 
with each of the signals, under various test 
conditions, are shown in Figures 6 through 23. 
The signal-to-background ratio found in this 
way for a 50 per cent recognition probability 
is called the recognition differential, usually 
abbreviated as RD. The RD is commonly ex- 
pressed as the decibel difference between over- 
all signal and overall background. Thus the 
recognition differential gives the relation be- 
tween overall signal and background levels at 
detectability. Since the filter analyses deter- 
mine the spectrum levels for the various sounds 
in terms of decibels below the overall level of 
the sound, the relation between the different 
frequency components of the signal and back- 
ground spectra is completely determined. Val- 
ues of the observed recognition differentials are 
given in the figure legends. 

From the preceding definition it is clear that 
the RD specifies a critical value of signal-to- 
noise ratio, and that the smaller the RD, the 
easier is detection. Since the detectable signal 
level is usually below background, the recogni- 
tion differentials tend to be negative ; any con- 


LEVEL IN l-CYCLE BAND IN OB 
ABOVE OVERALL BACKGROUND 


62 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 




4681 2 4681 2 4681 

100 1000 10,000 
FREQUENCY IN CYCLES 



OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 



100 


o 80 
(t 
< 

UJ 


60 


UJ 

40 
q: 

UJ 
Q. 


20 










n 





X 















/ 




f 















/ 





/ 

L 

E. EFF 

EOT OF FILTERS 

FILTER 

MD-PASS FILTER 












/ 


— NO 

0 — BAI 

L — 




— p— 

7 . 


V/ 


A 


/ 

f 


—4^... i-KC HIGH-PASS FILTER 


-30 


-20 -10 0 
OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 


10 


Figure 6. Audibility of sonic noise from aft planes (in power), masked by water noise. Character 
of signal: steady rapid knocking, with steady hum fluctuating slightly in pitch. 


RESIRICTED 


4 


BRITISH TESTS 


63 



100 1000 10,000 
FREQUENCY IN CYCLES 



100 


i 

tiJ 

^ 60 

o 40 
o: 

UJ 

a. 20 


0 

-30 -20 -10 0 10 

OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 


C. EFFECT OF PRESENTATION 

— random order 

9 — INCREASING LEVEL 

— 0 — INCREASING LEVEL, REPEATED 
□ INCREASING LEVEL, 2ND REPETITION 



■ .Tff 




!9 i 

k 






□ 

/ 

7 ) 

/ 










/( 

5 , 

7 / 



















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/* 

/ 















D-< 


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i 


9 














UJ 

X 


UJ 

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!S! 


60 

40 

20 

0 


E. EFFECT OF FILTERS 

1 



\ 

' 




-0— 



z 

-A— 

-tr— 

9 

— NO FILTER 

— BAND-PASS FILTER 

— I-KC HIGH -PASS FILTER 



/ r\ 

/ 

/ 

r 

f 

o 





/ 

f 




1 



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/ 

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-30 


-20 -10 0 
OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 


10 


Figure 7. Audibility of sonic noise from ballast pump, masked by water noise. Character of sig- 
nal: steady low-pitched hum and irregular crackling sound. 


HESTRICTED 



64 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 




100 1000 10,000 
FREQUENCY IN CYCLES 


100 

80 

60 

40 

20 

0 







/ 

7^ 

V ^ 



^ 
















/ 

/ 


C. EFFECT OF PRESENTATION 











i 

L / 

/ 

/ 

/ 



RAI 

rr 

NCX>M ORDER 

CREASING LEVEL 

REASING LEVEL, REPEATED 









, j 

^ j 

/ 

f — 

k 



__0 INC 







LJ 

bd 

r 

& 

















Q 

oc 

< 

UJ 

X 


UJ 

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(r 

UJ 

0. 


'30 


- 20 - 10 0 
OVERALL SIGNAL TO OVERALL NOISE RATIO IN OB 


10 



2 

X 


q: 


20 

0 * 






^ 

yi 



D—J7^ 

p— 

p 








y< 






/ 

/ 

r 

y 

/ 

1 

E. EFFECT OF FILTERS 

Tj NO FILTER 

__0_ BAND- PASS FILTER 

l-KC HIGH-PASS FILTER 



1 

/ 

r 


I 1 










/ 

/ 

1 

/ 

/ 





< 

r\ 

LN 

u 






/ 

/ 

/ 

/ 





L 


7 










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/ 

^ 




-30 


- 20 - 10 0 
OVERALL SIGNAL TO OVERALL NOISE RATIO IN OB 


10 


Figure 8. Audibility of sonic noise from forward planes (in power), masked by water noise. Char- 
acter of signal : medium-pitched hum, with steady pitch and fluctuating intensity. 


PER CENT HEARD PER CENT HEARD PER CENT HEARD 


BRITISH TESTS 


65 


eo 

a 


o 

z 

< 

OD 


y 


-I 



4681 2 4681 2 4681 


100 1000 10,000 


FREQUENCY IN CYCLES 





Figure 9. Audibility of sonic noise from starboard auxiliary circulator, masked by water noise. 
Character of signal : medium-pitched hum, with steady pitch and fluctuating intensity. A low- 
pitched hum was also audible. 


RESTRICTED 



PER CENT HEARD PER CENT HEARD PER CENT HEARD 


66 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 












■fl- 

fb-n 

D 





D 

3 

— 










/ 

// 


V 

/r 

t 
















/ 

/ 

/ 

/ 


/ 

/ y 

! 

/ 

& 


B. EFFECT OF PRESENTATION 

6-— RANDOM ORDER 

— -V INCREASING LEVEL 

__Q__ INCREASING LEVEL, REPEATED 
INCREASING LEVEL, 2ND REPETITION 







. 0 ^ 

• 



> 





— p- 

DD-P- 

Do-P- 



— p— 







OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 



OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 



OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 

Figure 10. Audibility of sonic noise from port motor cooling fan, masked by water noise. Char- 
acter of signal: low-pitched hum, with steady pitch and fluctuating intensity. Much water noise 
was also audible. 


LEVEL IN I -CYCLE BAND IN OB 
ABOVE OVERALL BACKGROUND 


BRITISH TESTS 


67 



100 1000 10,000 
FREQUENCY IN CYCLES 




-30 -20 -10 0 10 

OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 



-30 - 20 - 10 0 

OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 


10 



Figure 11. Audibility of sonic noise from lubricating oil separator, masked by water noise. Char- 
acter of signal ; medium-pitched hum, which rose in pitch and fell in intensity toward the end of 
the record. Also audible was a faint “ch ch ch” at 8 or 10 per second. 


68 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


dition which interferes with detection yields a 
smaller negative number (a larger RD). Since 
the ear has a fairly wide dynamic range, the 
absolute levels of signal and background are 


define the frequency bands in which signal and 
background are measured. If the intensities of 
the sounds fiuctuate, the time constant of the 
measuring instrument is a significant quantity ; 


S 

z 

0 

1 



6 8 I 
100 


2 4 6 8 1 2 

1000 

FREQUENCY IN CYCLES 


6 8 I 

10,000 




Figure 12. Audibility of sonic noise from lowering of aft periscope, masked by water noise. Char- 
acter of signal : sharp “ch” twice during the record. 


usually described in terms of subjective loud- 
ness only — for example, comfortably loud. Due 
to the fact that signal-to-background ratios at 
primaudibility are usually less than unity, the 
mixture of background and primaudible signal 
is only slightly louder, on the average, than the 
background alone. 

For the sake of precision, various quantities 
should be known accurately. It is necessary to 


so also is the point in the fluctuation cycle, for 
example, peak, mean or low, which is used to 
determine the level. The limits of deviation 
among observers and among tests should prob- 
ably be indicated. Similarly, the duration of 
each trial, the interval between trials, the total 
scope and duration of a test, and the subject’s 
level of experience, acuity, and fatigue should 
be known. 




BRITISH TESTS 


69 


Recognition differentials for the various sub- 
marine sounds shown here vary over a range of 
nearly 30 decibels. The extremes are illus- 
trated in Figures 8B {RD = + 5 db) and 16B 



FREQUENCY IN CYCLES 


observers taking the same test or for a given 
observer repeating a test. 

A much more reliable index of detectability 
is the difference in level between signal and 













/ t 



— u 

pv — 
















\ // 

/A 

/ 


















1 

1 

/ 

7/ 

/ / 



C. EFFECT OF LOUDNESS 










/ 

/ 

V 

/ 



— O ' 

90 PHONS 

70 PHONS 

50 PHONS 

o 




c 




b— 


i 



^ 

i 

b — A 


^ V 

> — ^ 



-30 


20 -10 0 
OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 



Figure 13. Audibility of speech transmitted by a submarine loudspeaker system, masked by water 
noise. Character of signal : words “six” to “seventeen”. The signal was intelligible although much 
water noise was audible. Spectrum A applies to “six,” spectrum B to “ten.” 


(RD = — 24 db). Even for a given signal 
under two different conditions of test (Figures 
8A and 8B — see Section 4.1.6 for a description 
of the tests), the RD was observed to change 
from —18 decibels in one case to +5 decibels 
in the other. This high degree of variability is 
not due to erratic behavior of the ear, since the 
scatter among the recognition differentials was 
rarely more than 1 to 2 decibels for different 


background spectra at the frequency where 
these spectra most closely approach each other. 
Thus, it will be observed that the total range of 
variation in this quantity is 10 decibels. The 
limits of variation are illustrated in Figures 7 
and 9, where the difference in level at the fre- 
quency of closest approach is 0 decibel, and in 
Figure 18, where the spectra are spaced about 
10 decibels apart at the point of closest ap- 


/^STRIGTED J 


70 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


-20 
-30 
-40 

-50 
-60 

-70 

4681 2 4681 2 4681 4681 2 4681 2 4681 

100 1000 10,000 100 lOOO 10,000 

FREQUENCY IN CYCLES FREQUENCY IN CYCLES 






O 

a: 

< 

UJ 

X 


z 

UJ 

o 

q: 

Ul 

Q. 


OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 



Figure 14. Audibility of sonic noise from port propeller (124 rpm), masked by water noise. This 
submarine proceeded at approximately 2.5 knots when both propellers were operated at 124 rpm. 
Character of signal : “ch ch ch” at 6 per second, and “swoosh” at 2 per second. The two modulation 
periods correspond to the blade and shaft rates, respectively, of a 3-bladed propeller. Also audible 
was a medium-pitched hum with steady pitch and variable intensity; a low-pitched hum was just 
audible. 


PER CENT HEARD PER CENT HEARD PER CENT HEARD 


BRITISH TESTS 


71 



100 1000 10,000 
FREQUENCY IN CYCLES 



OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 




-30 


20 


10 


0 


10 


OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 


Figure 15. Audibility of sonic noise from main motors in series (120 rpm), masked by water noise. 
Character of signal: high-pitched hum with steady pitch and variable intensity; a low-pitched hum 
was just audible, but the signal contained no noticeable propeller noise. 


r 




RESTRICTED 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 




Figure 16. Audibility of sonic noise from both propellers at 200 rpm (4.5 to 5 knots), masked by 
water noise. Character of signal: propeller thrash. 



FREQUENCY IN CYCLES FREQUENCY IN CYCLES 



Figure 17. Audibility of sonic noise from one propeller at 200 rpm and periscope depth, masked 
by water noise. Character of signal : propeller thrash, but slower and more musical than the sig- 
nal in Figure 16. 


BRITISH TESTS 


73 



z 

a 


? 

z 

Cj 



FREQUENCY IN CYCLES 


UJ 

X 


z 

cr 

UJ 

OL 


OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 



Figure 18. Audibility of sonic noise from main motors and propellers at 160 rpm (approximately 
3.5 knots) and periscope depth, masked by water noise. Character of signal: propeller thrash and 
a steady hum of medium pitch. 



4 6 8 1 

100 


4 6 8 1 2 

1000 


4 6 8 1 

10.000 



4681 2 4681 2 4681 

100 1000 10,000 


FREQUENCY IN CYCLES 


FREQUENCY IN CYCLES 


Q 

<r 

< 

UJ 

X 


tr 

UJ 

Q. 



OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 


Figure 19. Audibility of sonic noise from gears at 90 rpm, masked by water noise. Character of 
signal : pulsating complex hum, together with medium and high pitched tones. 



74 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 



CD _ 
Z 3 


UJ 

- UJ 


> o 



100 1000 10,000 
PREOUENCY IN CYCLES 



-30 -20 -10 0 10 


OVERALL SIGNAL TO OVERALL NOISE RATIO IN DB 

Figure 20. Audibility of sonic noise from gears at 60 to 70 rpm, masked by water noise. Charac- 
ter of signal : complex hum, fluctuating in pitch and intensity. 



100 1000 10,000 
FREQUENCY IN CYCLES 




Figure 21. Audibility of sonic noise from the ballast pump, masked by water noise. Character 
of signal: steady sound of medium pitch. 


RESTRICTE 


BRITISH TESTS 


75 


O 2 
m CD 

U 


ys 


-20 

•30 

-40 

-50 

-60 

-70 





A. N 

ON-DIR 

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1 1 THRESHO 

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6 8 1 2 4 6 8 1 2 

100 1000 

FREQUENCY IN CYCLES 


6 8 I 

10,000 


CDO -20 

Oz 

ii'“ 

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-50 


2) m 

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5 

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1 

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1 1 

2 

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100 


1000 

FREQUENCY IN CYCLES 


10,000 



Figure 22. Audibility of sonic noise from circulating pumps at full speed, masked by water noise. 
Character of signal: faint hum, rushing, and gurgling noises, with no predominant characteristic. 



4681 2 4681 2 4681 

100 looo loipo.o 

FREQUENCY IN CYCLES 



0 

(T 

< 

UJ 

1 


UJ 

o 

(T 

Ul 

Q. 



OVERALL SIGNAL TO OVERALL NOISE RATIO IN OB 


Figure 23. Audibility of sonic noise from circulating pumps at half speed, masked by water noise. 
Character of signal : steady hum of high pitch and gurgling noises. 


RESTRICTE 




76 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


proach. Use of the latter criterion instead of 
the RD reduces the variation from a factor of 
1,000 (30 decibels) to a factor of 10. The fol- 
lowing tabulation shows that the signal-to- 
noise ratios at the frequencies of closest ap- 
proach are better criteria of signal audibility 
than are the recognition differentials referred 
to a wide band. 




Signal-to-noise 


Signal-to-noise 

ratio in db, at 


ratio in db, in 

frequency of 


0.1- to 10-kc 

closest approach 


band 


Mean of 27 measure- 


ments 

— 9.5 

— 5.5 

Average deviation 

from 


mean 

4.3 

2.5 


These averages apply to all the spectra shown 
in Figure 6 through 23, except for the 5 high- 
pass filter cases. The latter are somewhat more 
difficult to interpret than the others; their in- 
clusion would have little effect on the tabulated 
values. In other words, the ability to hear a 
distributed signal in the presence of a distrib- 
uted background does not depend primarily on 
the total energy contents of the sounds. It is 
sufficient for detection that the energy content 
of the signal in an audible and relatively narrow 
frequency band approach the energy content of 
the background in that same frequency band. 
This amounts essentially to a restatement of 
the critical-band criterion for the audibility of 
tones in the presence of distributed back- 
grounds. Actually, it is an extension of that 
rule to the case in which the signal is not a 
pure tone, and in which both signal and back- 
ground show a high degree of fluctuation, or in- 
termittence. This extended critical-band rule 
implies, in addition, that the events in any one 
critical band are, to a first approximation, in- 
dependent of the events in any other critical 
band, and that recognition of a broad-band sig- 
nal may actually occur at a single optimal fre- 
quency or within a very narrow optimal fre- 
quency band. Of course, if the spectra of all 
underwater sounds had the same shape and 
time pattern, the recognition differentials would 
be identical for all signals in the presence of 
all backgrounds. 

This concept is of fundamental importance. 
If it is an adequate statement of what deter- 
mines the detectability of underwater sounds 


received in listening gear, it supplies the answer 
to a large number of operational and design 
problems. In addition, it furnishes a guide to 
the proper means of obtaining answers to a 
variety of still unsettled problems. To test the 
validity of this concept, a number of inferences 
from it may be examined ; in other words, the 
status of its validity may be analyzed in terms 
of a small number of related questions which 
have been or can be settled experimentally. The 
general nature of these subsidiary questions 
and the answers thereto are outlined in the 
following paragraph and also at various points 
in the remainder of this section; a more ex- 
tended discussion of the details will be found in 
Section 4.2.2. 

Perhaps the first question which comes to 
mind is this : are optimal frequency bands, that 
is, groups of signal frequencies which are solely 
responsible for signal detection, actually ob- 
served? This seems to be the case, as shown in 
Section 4.2.1. Secondly, why, if the critical- 
band concept is valid, are the spectra separated 
by as much as 10 decibels at the optimal fre- 
quency (point of closest approach) ? From the 
critical-band rule for tones, signal and back- 
ground energies would be expected to be equal 
in the optimal critical band. There are at least 
two factors which contribute to this apparent 
departure from the critical-band rule: (1) fluc- 
tuation in signal and background levels and (2) 
obliteration of detail in the spectra caused by 
using filters which are wider than the critical 
bands. Thus, the peak signal levels were ob- 
served to rise 3 to 5 decibels above the mean 
levels which were used in determining the 
relation existing between signal and background 
spectra when the signal was just audible. It is 
probable that the signal was detected at these 
peak levels. In addition, if the levels of the 
various frequency regions in the spectra had 
been determined with narrower filters, these 
levels would probably be some 5 decibels higher 
than those shown in the neighborhood of the 
audible tones indicated in the various figures. 

This estimate is reached in the following way. 
Suppose a pure tone of frequency / and in- 
tensity It is mixed with a wide-band distrib- 
uted sound whose average intensity per cycle is 
I , and the energy of the mixture is measured 



BRITISH TESTS 


77 


through each of two filters with the same mid- 
frequency /, one of the filters having a pass 
band Af^ equal to the width of a critical band 
at the frequency / and the other having a pass 
band Afoct equal to the width of an octave cen- 
tered at the frequency /. What would be the 
ratio of the “spectrum levels” of the mixture, 
as deduced from measurements with the two 
filters? The values of these spectrum levels may 
be obtained as follows. 

1-cycle spectrum level obtained with a criti- 
cal-band filter. 

la. Total intensity 

I cb = I T I-^Afcb 

2a. Average intensity per cycle 

I cb It I^Afcb 

Afcb ~ Afcb 

1-cycle spectrum level obtained with an 
octave filter. 

lb. Total intensity 

loct = It I ^ Afoct 

2b. Average intensity per cycle 

loct _ It -j- I ^ Afoc t 

^foct ^foct 

By dividing 2a by 2b to obtain the ratio R of 
average intensities deduced from the narrow 
and wide filters and by denoting {It/I-) by K, 
we obtain after simple manipulation, 

10 log R 

This is the difference, in decibels, between the 
spectrum levels obtained by the two methods of 
measurement. 

The value of this difference in level will be 
estimated for several representative cases. Con- 
sider a tone at 1 kilocycle, where Afcb — 50 
cycles, and Afoot = (%) 1,000 cycles. If the in- 
tensity of this tone were 100 times as great as 
that of the distributed sound in a 1-cycle band 
in the signal {K = 100) , the tone would be an 
audible part of the signal when no background 
was present but would not be relatively strong. 
On the other hand, if the intensity of the tone 


were 1,000 times greater than that of the sig- 
nal, in a 1-cycle band, the tone would be rated 
strong, when the signal was heard in the ab- 
sence of the background. The value of 10 log R 
for these two cases is computed below (see also 
Figures 59 and 60). 

lOlogfl = 10 log 0 ~ 0 

= 4.3 db (2) 

10 log R = \0 + l) - 10 log + l) 

= 9.2 db. (3) 

For an exceedingly strong tone, 10 log R would 
approach its limiting value 10 log Afoct/^fcb, 
amounting in this case to 11.2 db. 

Hence, the presence of an audible tone at 1 
kilocycle in the signal implies that the signal 
spectrum would in general be at least 4 decibels 
higher if the measurements had been performed 
with critical-band filters. The major effect 
produced by the use of the octave, rather than 
critical-band filters, is this reduction of level at 
the frequencies where tonal components occur ; 
a correlated effect is the elevation of regions 
remote from the tone frequency, because sev- 
eral of the octave filters were able to pass the 
tone. 

When the optimal component in the signal is 
a tone with a frequency in the neighborhood of 
125 cycles, the width of the octave filter cen- 
tered at the tone frequency is approximately 
equal to the width of the critical band stimu- 
lated by the tone; in other words. Afoot = (%) 
125 cycles, whereas Afch is about 45 cycles in 
this frequency region. Under these circum- 
stances octave filters give adequate resolution 
of the test sounds, and the tone-to-noise ratio 
should be approximately unity when the signal 
is just detectable. This situation is illustrated 
in Figures 7 A, 9 A, and 22 A. When the tone has 
a somewhat higher frequency so that Afoot> 
Afcb, the spectra of signal and background tend 
to be spaced by some 5 to 10 decibels at the fre- 
quency of closest approach. These situations 
are illustrated in Figures 8 A, 11 A, 15 A, and 
20A. 

In some cases, the masked signal becomes 
audible as a band of frequencies. The spectrum 


78 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


deduced from octave-band measurements then 
tends to be fairly comparable to what would be 
obtained from critical-band measurements and, 
provided the wide-band component responsible 
for signal detection does not exhibit a strong 
modulation, detection would be expected to 
occur when signal and background spectra coin- 
cide at the optimal frequency. This situation is 
illustrated in Figures 13A, 13B, and 22B, and 
also in Figures 7B, 8B, IIB, and 14B. 

Finally, when the masked signal is detected 
as a wide band of frequencies, that is, when 
signal and background are parallel over a large 
frequency interval in the optimal region, and 
the optimal component has a strong and char- 
acteristic modulation, as in the case of propeller 
cavitation, detection depends essentially on the 
ability to discriminate changes of intensity 
rather than changes of quality. Under these 
conditions, recognition would be expected when 
signal and background spectra are spaced by 
5 to 9 decibels at the frequency of closest ap- 
proach. The basis of this estimate is discussed 
in Section 4.2.3 ; illustrative cases are shown in 
Figures 12A, 16A and 16B, 17A and 17B, and 
18B. From these figures it will be noted that 
the observed spacing between signal and back- 
ground is 6 to 10 decibels, for modulation rates 
in the neighborhood of 3 cycles, in excellent 
agreement with expectation. 

The general conditions which must be met in 
order to assure aural detection of various types 
of sounds masked by specific kinds of noises are 
summarized in Chapter 6. It is worth noting 
some related practical matters at this point, 
since these aspects of the general problem do 
not depend essentially on the further experi- 
mental details given below. In the first place, 
it is obvious that the measurement of ship 
spectra at maximum detection distances is not 
feasible, since the signal levels, except in the 
optimal detection band, are usually far below 
background. Secondly, the submarine noise 
analyses given here are not entirely satisfactory 
from either a scientific or a practical stand- 
point, since they represent measurements on a 
small number of vessels, none of them current 
American fleet-type submarines, and the analy- 
ses were made with wider filters and slower 
recording instruments than seems desirable. 


Nevertheless, they are the only detailed sub- 
marine machinery spectra available at the 
present time. A few narrow-band measure- 
ments on the machinery sounds produced by 
fleet-type submarines have been made recently, 
but the results of these tests have not yet ap- 
peared in usable form. Another point which 
should be mentioned is this: enemy listening 
gear may be able to detect occasional, or highly 
intermittent sounds (see Figures 12 and 13) 
when they occur, but these may be inadequate 
for maintaining contact with a target. 


Transition Curves 

The process of signal detection and its rela- 
tive efficiency are affected by a number of con- 
ditions which enter into the test situation and 
into the practical situation as well. In this 
series of tests, the following factors were 
varied, one at a time : the sequence in which the 
different signal levels were presented, the total 
loudness of the mixture, and the effect of filters. 
The influence of each of these factors is best 
shown by the group of transition curves which 
are included in the various figures. The various 
transition curves are shown together with the 
spectrum of the signal which was used in 
making the test, and the titles attached to the 
transition curves indicate the factor whose 
effect is depicted by the curve. The discussion 
of each of these variable factors, based on an 
examination of these transition curves, is given 
in the following four sections. Certain general 
considerations about transition curves are 
presented here first. 

The transition curves, as their name implies, 
show that the emergence of recognizable dif- 
ferences among successive mixtures of sounds, 
as a function of their relative intensities, is not 
infinitely sharp. The probability that a signal 
will be detected increases gradually from nearly 
zero to nearly unity over a range of some 10 
decibels. The standard procedure of taking the 
50 per cent point as the signal-to-background 
ratio at which detection occurs is a reasonable 
approximation in many cases, but it is not 
equally appropriate to the analysis of all tacti- 
cal situations. The practice has therefore been 


BRITISH TESTS 


79 


to define the precise detection probability which 
is intended in a particular discussion, for exam- 
ple, 50 per cent RD, 80 per cent RD. 

For present purposes, it is sufficient to draw 
attention to the fact that the transition curves 
obtained in various tests are an important sup- 
plement to the information obtained from the 
spectra of the sounds used, and that they should 
be consulted along with the other relevant data 
if a detailed understanding of the detection 
process is desired. It is plain, therefore, that 
such curves should be described together with 
other results of listening studies. 

The information supplied by a significant 
transition curve is the probability of perception 
under given circumstances. It is consequently 
important that the observers have the proper 
attitude, in other words, that they avoid guess- 
ing as well as excessive caution in reporting 
faint signals. Each of these kinds of bias tends 
to produce a characteristic distortion of the 
typical transition curve, since each deviates 
from the curve of perception by giving a pre- 
ponderance of errors, either of omission or of 
commission. Thus, the inversion in the transi- 
tion curve indicated by the circles in Figure 
7D and the extremely gradual decrease of per- 
ception with decreasing signal level shown by 
the curve indicated by the circles in Figure IOC 
are distortions commonly due to guessing. Al- 
ternatively, an unusually gradual rate of de- 
crease in detection probability with decreasing 
signal level may mean that some feature of the 
signal is particularly easy to recognize, and 
therefore detection is impaired less by a drop 
in signal-to-noise ratio than is ordinarily ob- 
served. Illustrations of a steeper than average 
slope, which often characterizes the curves 
given by excessively cautious observers, may 
be noted in Figures 13D and 19C. 

Since guessing falsifies the results of listen- 
ing tests, it is essential to eliminate all scores 
bearing evidence of this tendency, and, when 
observers show a persistent bias in this direc- 
tion, to eliminate them from the tests. A stand- 
ard device used to indicate this kind of un- 
reliability is to include in the test a group of 
presentations which contain only the back- 
ground sound ; if an observer reports the signal 
when it is absent, his performance is unreliable. 


This procedure of including “blank” presenta- 
tions was employed in the tests under discus- 
sion. The percentage of the blanks reported 
audible is shown by the symbols on the left- 
hand borders of those transition curves ap- 
pended to Figures 6 through 15 which are 
labeled “effect of loudness” and “effect of 
filters.” It will be observed that the number of 
false reports was vanishingly small except in 
the two cases already mentioned (Figures 7D 
and IOC) . Even here, this tendency appears for 
only one condition of test, and it seems to have 
originated from confusion rather than reckless- 
ness (see Section 4.1.5). 

There is no equally simple way of diagnosing 
an excessively cautious attitude on the part of 
the observer. In general, however, the transi- 
tion curves given by such observers are signifi- 
cantly steeper than those obtained from “aver- 
age” or “unbiased” observers taking the same 
test. Since transition curves obtained in labora- 
tory tests represent performance under un- 
usually good conditions, with distractions, bore- 
dom, and fatigue reduced to a minimum, it 
seems likely that a conservative attitude should 
be encouraged in the observers. This is partic- 
ularly true since there is no laboratory equiva- 
lent to the serious or, at least, embarrassing con* 
sequences which may follow a false report in 
the practical situation. It is not intended to 
imply that sound operators are overly cautious 
(they are, in fact, trained and instructed to 
report all doubtful contacts, and to describe 
them as such), but rather that the observer in 
most laboratory tests has the advantage of 
knowing that audible signals will be contained 
in the series of sounds presented to him. He is 
on the alert for signals and has no hesitation in 
identifying them as such because he knows that 
any difference in the sounds presented must 
be due to the signal. He need not stop to con- 
sider whether the faint and often fleeting 
change in character which he hears originates 
aboard his own vessel, for example. Further- 
more, to simplify the analysis of the data, most 
laboratory tests are neatly subdivided into a 
group of listening intervals each featuring 
either a signal or a blank. It is clear, therefore, 
that the type of laboratory test under discus- 
sion tends to define an upper limit of perform- 


80 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


ance, and it is important that this limit should 
be established. But it is equally important that 
a parallel study be made of the performance 
of average sound operators under typical field 
conditions (see also Section 4.2.6). 

There is one important aspect of these listen- 
ing tests which cannot be adequately repre- 
sented by the spectra, the time patterns, or the 
transition curves, namely, the subjective proc- 
esses of perception, memory, and judgment 
which are associated with signal recognition. 
Some insight into these subjective matters can 
be gained through discussion with experienced 
observers, or better yet, by participating in a 
few typical listening tests.^^ For example, the 
degree of subjective certainty with which a 
signal can be identified as ‘"audible” may often 
be fairly low. This is particularly likely to be 
true for faint signals (those with levels corre- 
sponding to less than 50 per cent detection 
probability) and for presentation methods in 
which the successive test sounds are widely 
spaced in time and not subject to control by the 
observer. Under these circumstances the ob- 
server must compare the sound just presented 
with a mental image of the masking back- 
ground in order to decide whether any change 
has occurred due to the introduction of a sig- 
nal; he may often pause for several seconds 
before arriving at a decision. As pointed out 
before, this decision may be influenced by the 
observer’s personal bias. Listening under these 
conditions requires a high degree of attention 
and concentration; hence, a protracted interval 
of such testing is fatiguing and irritating and 
often yields highly variable results. 

The problem of fatigue is met in practice by 
relieving the sound operator at fairly frequent 
intervals.® The particular procedure followed in 
this connection is probably somewhat variable ; 
since no systematic study of optimal procedure 
has been made, present practice is presumably 
based on a mixture of expediency and practical 


Much of the material in the present section is de- 
scribed here for the first time. The discussion is based 
largely on still unpublished observations made in lis- 
tening studies by University of California Division of 
War Research [UCDWR] and at the Mountain Lakes 
Station of CUDWR-USRL. The contents of these in- 
formal communications are introduced at the present 
point in order to unify the discussion of the problem. 


experience. It has been suggested that a signal 
generator, controlled from the bridge and ar- 
ranged to radiate an underwater signal of 
known strength to the receiving hydrophone, 
would be a useful aid in maintaining and check- 
ing the alertness of sound operators. Such a 
device should also be useful in helping to adjust 
the listening gear and to determine the extent 
of the masking background at different times 
and under various conditions of operation. 

Transition curves can in general be repre- 
sented by an expression involving two param- 
eters: (1) the signal-to-background ratio cor- 
responding to 50 per cent detection probability, 
and (2) a quantity n which is related to the 
steepness of the transition curve. This basic ex- 
pression may be written in the form 



where P is detection probability, S is signal in- 
tensity, and Sr^Q is the signal intensity corre- 
sponding to 50 per cent detection probability. 

It may be noted that the shape and steepness 
of the available transition curves are not 
markedly different for fluctuating and nonfluc- 
tuating signals, when either is masked by a 
fluctuating wide-band background. Hence, 
equation (4) applies about equally well to both 
types of masking. For both types of curve, the 
mean value of n is approximately 3. Extreme 
values of n are about 1.5 and 9. 

The exponent n is directly related to the 
“spread” of the transition curve, where this 
spread is defined as the number of decibels in- 
crease in signal-to-background ratio which is 
required to raise the detection probability from 
20 per cent to 80 per cent. Substituting in 
equation (4) the pairs of values for P and S 
appropriate to 20 per cent and 80 per cent 
detection probability, respectively, gives 


0.8 _ / ^50 Y 

0.2 ~ ~ \^2o/ ’ 

(5) 

0.2 1 /^soV 

0.8 4 \^*s8oy • 

(6) 


Dividing equation (5) by equation (6), and 
converting to decibel form, yields 




KESTRIGTEU 


BRITISH TESTS 


81 


fe) ■ « 

The parenthesis in the right-hand member of 
equation (7) is the quantity defined as the 
spread in the previous text. Hence, finally, the 
spread is 

= rA db. (8) 

n V ^ / 

Thus, the spread is 4 decibels when n equals 
3. Furthermore, the spread diminishes, that is, 
the steepness increases, as n increases. A range 
of n between 1.5 and 9 corresponds to a range 
of spreads between 8 and 1.3 decibels. It is con- 
venient to describe transition curves in terms 
of the spread as defined here because the nu- 
merical value of the spread is easily obtained 
by inspection; hence, n is readily evaluated 
with the aid of equation (8). Since also the 
“tails” of the transition curve (the intervals 
corresponding to detection probabilities of less 
than 20 per cent or more than 80 per cent) are 
usually not well defined, a purely observational 
quantity, such as the spread, is more appropri- 
ate than a theoretical parameter which defines 
the tails of the transition curve as well as the 
intermediate region. 

When examining these transition curves, it 
should be borne in mind that they are com- 
posite curves obtained by lumping together the 
responses of all the observers in the listening 
group; slightly different results are obtained 
by averaging the number of signals detected by 
the entire group at a given signal-to-back- 
ground ratio and by averaging the signal-to- 
background ratios required to give a particular 
detection probability for all observers. Fur- 
thermore, different observers taking the same 
test sometimes give transition curves with 
rather widely different values of n. In general, 
however, such differences tend to be minor. The 
various RD values obtained with a group of ex- 
perienced observers rarely differ by more than 
1 to 2 decibels. 

The transition curves given by experienced 
observers are found to differ somewhat from 
those obtained with inexperienced observers, as 
the following data show. Ten observers were 
employed for that part of the tests under dis- 
cussion, which are described in Sections 4.1.5 


and 4.1.7. The five most consistent and reliable 
members of this group of ten were used in the 
remaining parts of the test program. The 
major difference between the transition curves 
obtained with the large and the small group 
was the greater tendency toward errors of com- 
mission shown by the less experienced ob- 
servers; the cleaner “yes-no” transition char- 
acteristic of the probability curves in Figures 
16 through 23 is typical of the more reliable 
observers. There was no significant difference 
in the RD values obtained with reliable ob- 
servers and with observers inclined toward 
guessing. The mean difference in recognition 
differentials among the group of five consistent 
listeners was 0.2 decibel, although in one case 
(see Section 4.1.5) a member of this group 
failed to detect the signal when it was 6 decibels 
higher in level than the remaining four ob- 
servers required for 50 per cent detection 
probability. 

Several operational conclusions are indicated 
by the preceding discussion. The 50 per cent 
points are useful guides in computing general 
performance, but in cases where it is desired to 
estimate the signal levels corresponding to 
nearly certain detection, 6 decibels should be 
added to the 50 per cent signal-to-background 
ratio, and similarly, 6 decibels should be sub- 
tracted from the 50 per cent value to estimate 
the level of undetectable signals. If n is set 
equal to 3 in the transition curve equation and 
the value of the term 10 log S/S^^ is either in- 
creased or decreased by 6 decibels, the corre- 
sponding values of detection probability are 
found to be 98.5 per cent and 1.5 per cent re- 
spectively. The spread in the transition curve 
indicates that, as the tactical situation varies 
from one condition to another, a detection 
probability greater or less than 50 per cent 
may be more significant than the average or 
50 per cent detection probability. In addition, 
many targets may be first detected by means 
of prominent tonal components in their acoustic 
outputs. Such tonal components may not be suf- 
ficiently characteristic to permit identification 
of the target and will almost certainly be of no 
help in making a propeller turn count. In such 
cases, the range would have to be closed to ob- 
tain further information by means of listening. 


10 log 16 


3 = 


10 log 


82 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


4.1.5 Effect of Presentation Method 

The effects of two different methods of pre- 
senting the test sounds were examined. In both 
these methods, the level of the masking back- 
ground was maintained constant (after it had 
been adjusted to a loudness level of 70 phons), 
and the signal was introduced at various levels 
which differed by at least 2 decibels and cov- 
ered the range from inaudible to audible. In 
both methods, also, the mixture of signal and 
background was presented for about 15 seconds, 
and a silent interval of 3 or 4 seconds separated 
successive presentations in order to allow time 
for recording signal audibility. The chief dif- 
ference between the two methods of presenta- 
tion was the fact that successive signal levels 
were presented in random order in one case, 
but in order of gradually increasing intensity 
in the other case. The random order method 
was intended to minimize the effects of guess- 
ing (and therefore included blank presenta- 
tions) ; the gradually increasing level method 
was intended to simulate the field situation 
which might occur when the range to a sub- 
marine is gradually closed (and therefore in- 
cluded no blank presentations). The results are 
given in those transition curves in Figures 6 
through 15 which are labeled “effect of presen- 
tation.’' These results seem to demonstrate, as 
indicated below, that there is no significant dif- 
ference between the two methods (in the labo- 
ratory, and for wide-band listening) . They dem- 
onstrate also that the observer should make 
a conscious effort to examine all the frequencies 
in the presentation band.® 

Random Order Tests 

Ten observers were used in the random order 
tests, which preceded the tests with increasing 
signal level ; the background was maintained at 
a level of 70 phons. As preparation for the ran- 
dom order tests, the observers were presented 
with the background alone, with the signal 
alone, and finally were permitted to listen to 

® It is convenient to distinguish between the standard 
reference band which extends from 0.1 to 10 kilocycles 
and the presentation hand which is the group of fre- 
quencies actually presented to the listener. These fre- 
quency bands do not necessarily have the same limits. 


the mixture while the signal gradually faded 
out. Each set of tests was performed twice, the 
order being changed in the duplicate test. 

The transition curves give the average re- 
sult. Since no mention is made of significant 
changes in performance on successive tests 
with a given signal, it may be assumed that 
practice effects were negligible. “It was found 
desirable,” according to the report^ describing 
the results of these tests, “not to make the 
order completely random, but to avoid the first 
level presented being a borderline case and to 
have an easily heard level early in the series. 
If the series begins with a number of ‘not heard’ 
observations the observers tend to be discour- 
aged, and are more inclined to give random 
‘heard’ observations at very low levels.” This 
procedure seems a very poor approximation to 
the field situation, where the operator may fail 
to obtain a single contact through the entire 
course of a watch or, indeed, many consecutive 
watches. It is therefore desirable to simulate 
the practical condition in a small number of 
laboratory tests, and this has been done (see 
Section 4.2.6). Obviously, the expenditure of 
time required to conduct all tests in this man- 
ner would be prohibitive. Furthermore, it would 
be surprising if the change in testing procedure 
produced a large effect on the results. Nonethe- 
less, it is useful to know what allowance, if 
any, should be made in applying laboratory re- 
sults obtained by standard methods to the prac- 
tical situation. 

Increasing Signal-Level Method 

The five most reliable observers were tested 
by the increasing signal level method. The re- 
sults for these same observers by both methods 
of test are shown in the transition curves 
(Figures 6 through 15) labeled “effect of pres- 
entation.” Comparison of the transition curves 
given by these five observers under the random 
method of test with the transition curves given 
by the entire group of ten observers for the 
random method, as shown in the curves labeled 
“effect of loudness,” indicates that the five ex- 
perienced observers gave recognition differen- 
tials which were not more than 0.4 decibel 
lower, on the average, than those obtained with 
the entire group of ten. 




f { RESTRICTED 




BRITISH TESTS 


83 


In this method of test, the background alone 
was presented first. Then one of the signals 
previously used but not otherwise identified 
for the observers was introduced at a level 
which, from the earlier tests, was expected to 
be inaudible; the signal level was subsequently 
increased in steps of 2 decibels. The observers 
were requested to distinguish between certain 
and doubtful perceptions of the signal in order 
to check guessing. A maximum of two ‘*pos- 
sibly heard’^ reports was accepted, on the as- 
sumption that if an increase of 4 decibels in 
signal level was insufficient to produce a large 
increase in subjective certainty, the “possibly 
heard’’ report should have no weight. In gen- 
eral, little trouble was encountered from this 
source (the observers’ knowledge that each 
presentation contained the signal) and the 
character of the transition curves obtained by 
both methods of test is substantially the same. 

It is significant that in no case was the RD 
larger (or recognition more difficult) for the 
increasing-level method than for the random- 
order method and that in four of the eight 
cases studied by both methods there was a sub- 
stantial improvement in the observers’ ability 
to detect faint signals when the increasing level 
method was used. The evidence indicates that 
this improvement was real, that is, not owing 
to practice effects or other spurious causes. It 
also indicates that the improvement depended 
on alertness rather than on the presentation 
method per se. 

Thus, in the random-order tests, the fact 
that the observers had foreknowledge of the 
character of the signal sound when it was 
not masked — either because it was presented 
alone or because it was presented together with 
the background but at a relatively high level 
— predisposed them to listen for the most char- 
acteristic features of the signal during the 
masking tests. Such foreknowledge is not 
typical of search conditions in the field. Fur- 
thermore, it is irrelevant and misleading in 
masking studies because the more striking 
aspects of a wide-band signal are not neces- 
sarily preserved when that signal is just audible 
in the presence of a masking background, in- 
asmuch as the optimal component of the 


primaudible signal may not be a distinctive 
feature of the pure signal. 

It was found during the course of these 
studies that if the signal were presented alone 
at a comfortable loudness, and then gradually 
faded out, with no masking background 
present, its character remained very much the 
same except for those signals containing a low- 
pitched hum. For these sounds the low-pitched 
component disappeared first, as would be an- 
ticipated from the fact that the sensation levels 
(number of decibels above the audibility 
threshold) at the lower frequencies are gen- 
erally smaller than at the higher (see, for ex- 
ample, Figures 7, 9, 10, and 14) . Hence, when 
the overall level of such a sound is diminished, 
the low-frequency components will be reduced 
to inaudibility before the higher frequencies 
fall below threshold. It was found also that, in 
the presence of the masking background, the 
character of the signal often changed dras- 
tically when its intensity was reduced toward 
the masked threshold while the background 
level was held constant. Those features which 
were most characteristic of the signal when 
heard alone were the first to be lost in the 
presence of the background, for example, the 
rapid knocking of the aft planes (Figure 6), 
the sharp crackle of the ballast pump (Figure 
7), and the propeller thrash of the slow, sub- 
merged submarine (Figure 14). These are 
high-frequency sounds; thus, a masking back- 
ground may impair signal audibility in a quite 
different frequency region from that affected 
by the auditory threshold. In all cases (except 
speech. Figure 13, and propeller thrash. Fig- 
ures 16, 17, and 18) the primaudible signals 
were finally detected as more or less steady 
hums or tones; this was found to be the case 
with either presentation method. 

Thus, the improvement obtained with the in- 
creasing-level method of presentation was ap- 
parently due to the fact that the observers were 
not listening most efficiently in this particular 
set of random-order tests. For efficient listen- 
ing, it is necessary to search actively every part 
of the presentation band for traces of the sig- 
nal, and not merely to attend passively to the 
entire presentation band or to a restricted fre- 
quency interval within which the signal is ex- 


84 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


pected to occur. Active search was encouraged 
in the increasing-level tests by the fact that the 
character of the signal was not known to the 
observers in advance of the test. Table 1 com- 
pares the recognition differentials obtained 
with both presentation methods for the cases 
in which this factor made a significant differ- 
ence. It will be noted that only four of the 
eight signals gave results which depended on 
presentation method, and that, when the ob- 
servers learned to listen effectively, the im- 
provement was maintained upon repetition of 
increasing-level trials. In other words, the 
change was not due to instability in the ob- 


7), for example, one observer failed to detect 
the signal until it reached a level 6 decibels 
higher than that at which it had been detected 
by the other four. Thus, the use of a 6-decibel 
“safety factor,'’ which was suggested at the 
end of Section 4.1.4 for the sake of assuring 
near certainty of detecting a given frequency 
component in the signal, seems warranted for 
the additional reason that a sound operator 
may overlook the optimal component. It seems 
clear from these observations that the cue 
which permits detection of the faintest signal 
does not necessarily thrust itself upon the at- 
tention. A diligent search may often be needed 


Table 1. Effect of presentation method. 


Signal 

Transition 

curve 

Recognition differential in db 

Approximate 
improvement 
in audibility 
in db 

Random- 

order 

tests 

In 

1st trial 

creasing-level tes 

2nd trial 

its 

3rd trial 

1. Ballast pump 

Figure 7C 

-12 

-12 

-14 

-15 

2 

2. Forward planes 

Figure 8C 

— 19 

-21 

-21 


2 

3. Cooling fan 

Figure lOB 

- 8 

- 9.5 

-15 

-13.5 

6 

4. Port propeller 

Figure 14C 

- 9 

-12 

-15 

-14 

5 


servers’ reactions. However, if the random- 
order tests had followed the increasing-level 
tests, rather than preceding them, it is possible 
that a different result would have been ob- 
tained. 

A given wide-band signal, masked by a given 
background, may be detected at different fre- 
quencies by different observers, or by the same 
observer at different times. That band of fre- 
quencies which permits detection of the faint- 
est primaudible signal, in the presence of a 
particular background, is called the optimal 
frequency hand. Failure to respond to the opti- 
mal signal frequency may require an increase 
of 6 to 8 decibels in the level of the primaudible 
signal (see Table 1). It is significant that the 
three signals (Figures 7, 10, and 14) for which 
there was the greatest variation between the 
results obtained with different presentation 
methods were the signals which also gave the 
greatest variation in results for individual ob- 
servers. With the ballast pump signal (Figure 


to find it, and failure to look for such a cue 
may be expected to yield results no better than 
would be obtained if the optimal cue were 
absent or suppressed.^ Therefore the develop- 
ment of proper search habits should probably 
be emphasized in the training of sound opera- 
tors. This need for careful examination of the 
received sounds, as well as random changes in 
the levels of signal and background, impose a 
lower limit on the time that an operator should 
allot to the output from a given hydrophone 
or from a hydrophone trained on a given 
bearing. 

Failure of an observer to respond to the op- 
timal cue, even after the signal-to-background 
ratio is substantially increased above the values 

^ It is interesting to compare the transition curves 
listed in Table 1 with those illustrating the effect of 
filters which attenuate the optimal frequency band (see 
Section 4.1.6). Careless search and actual suppression 
have essentially the same effect; the transition curves 
are shifted, with no significant change in shape, to 
higher values of the signal-to-background ratio. 


RESTRICTED 


BRITISH TESTS 


85 


required by other observers, may be due either 
to confusing similarity between the optimal 
cue and the background or to unfavorable pres- 
entation. Both factors seem to have played 
some part in the tests under discussion. For 
example, the ballast pump (Figure 7) and the 
cooling fans (Figure 10) are predominantly 
low-frequency signals, and were considered by 
the observers to be the most difficult to detect 
with certainty. Examination of the spectra for 
these cases indicates that the frequencies below 
200 cycles were not far above the estimated 
threshold, whereas the background frequencies 
above this value were presented at consider- 
ably higher sensation levels. Hence remote 
masking of the optimal signal band, or at least 
distraction due to the background components 
at higher frequencies may easily have occurred. 
This phenomenon is especially likely because 
acoustic leakage around the headphone cap, 
with consequent reduction of intensity, is a 
significant problem for these lower frequencies. 
Furthermore, it was observed that the RD for 
the cooling fan was considerably smaller when 
the background was presented at a level of 90 
phons ; in other words, increasing the loudness 
of the low-frequency components improved the 
observers' ability to detect the optimal cue. 

The spacing between signal and background 
spectra in Figures 10 and 14 would be in- 
creased by 5 to 6 decibels if the results of the 
increasing-level tests had been used rather 
than those of the random-order tests. However, 
the spectra were not analyzed with sufficient 
precision to permit any very significant conclu- 
sions to be drawn from this change. 


Effect of Filters 

Electrical filters were inserted between the 
mixer and the mixture amplifier (Figure 1), 
thereby passing or suppressing the same fre- 
quencies in signal and background. This pro- 
cedure simulates the effect of filters in practical 
installations. Two types of filters were used: 
( 1 ) a high-pass filter which transmitted all the 
frequencies higher than 1 kilocycle and which 
had a fairly sharp cutoff below that frequency 
and (2) various octave band-pass filters which 


were intended “to pass the predominant signal 
frequencies." The predominant frequencies 
were determined by examination of the signal 
analyses, plotted as energy per octave. Such a 
basis for choice is unsatisfactory for two rea- 
sons: (1) the ear’s critical bands do not cor- 
respond to octaves, and (2) the optimal fre- 
quency band depends on the relative levels of 
signal and background spectra, and not pri- 
marily on the absolute level at various fre- 
quencies in the signal spectrum. Despite these 
objections the filter tests provide useful infor- 
mation. 

These tests were carried out with the signals 
and background shown in Figures 6 through 
15. The random-order method of presentation 
was followed, and the same group of five most 
reliable observers was used as in tests pre- 
viously described. All the tests were made at 
a listening level of 70 phons. Since the filters 
excluded large parts of the standard reference 
band from the ears of the observers, the sensa- 
tion levels at the transmitted frequencies were 
higher than in the tests in which the entire 0.1- 
to 10-kilocycle band was presented at a level of 
70 phons. To the extent to which auditory 
acuity improves with increasing sensation level, 
the presentation of the optimal frequencies 
at a higher sensation level should permit the 
detection of fainter signals, and the results 
seem to show that this factor played a small 
but noticeable role in this group of tests. 

Table 2 lists the improvement in detect- 
ability produced by the insertion of the various 
filters. Improvement is here defined as the 
difference between the RD without filter and the 
RD with filter. Thus, if the RD with filter is a 
larger negative number than the RD without 
filter, the difference is positive ; in other words, 
there is an improvement or increased ability to 
detect weaker signals. Conversely, a negative 
difference indicates a deterioration due to the 
use of the filter. The recognition differentials 
(decibel difference between primaudible signal 
and background in the standard reference band 
at the input to the filter) are used rather than 
the presentation differentials (decibel differ- 
ence between primaudible signal and back- 
ground at the output of the filter) because per- 
formance in the field is best expressed in terms 


lESTRIGTED 


86 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


of the sounds-in-the-water rather than the 
sounds-at-the-ear. Similarly, the pertinent 
transition curves, which are listed in Table 2, 
all show the signal-to-background ratio at the 
inputs to the filters. All these transition curves 
represent the performance of the same five 
observers for various conditions of test. 


loss brought about by failure to make a careful 
search for the optimal cue in this particular 
signal (line 4 of Table 1). It is possible, there- 
fore, that the optimal component in the case 
of the port propeller occurred in the neighbor- 
hood of 200 cycles. The primaudible signal, in 
the case of the ballast pump, was difficult to 


Table 2. Effect of filters. 




Band-pass filters 

High-pass filter 

Signal 

Transition 

curve 

Passband 
in cycles 

Improvement 

in 

audibility 
in db 

Transmitted 
spectra at 
primaudi- 
bility 

Improvement 

in 

audibility 
in db 

1. Aft planes 

Figure 6E 

220-440 

0 

Figure 6B 

- 9 

2. Ballast pump 

Figure 7E 

110-220 

-2 

Figure 7B 

-14 

3. Forward planes 

Figure 8E 

165-330 

2 

Figure 8B 

-24 

4. Auxiliary circulator 

Figure 9D 

440-880 

1 


- 2 

5. Cooling fan 

Figure lOD 

110-220 

1 


Not detected 
at any level 

6. Lubricating oil separator 

Figure HE 

110-220 

1 

Figure IIB 

-14 

7. Aft periscope 

Figure 12C 

2,400-4,800 

1 


0 

8. Speech 

Figure 13D 

1,300-2,600 

0 


0 

9. Port propeller 

Figure 14E 

440-880 

-4 

Figure 14B 

- 3 

10. Main motors 

Figure 15D 

660-1,320 

0 


- 2 


The third column of Table 2 gives the limits 
of the octave filters which were used in these 
tests because they passed the ''predominant” 
signal frequencies. These same limits are indi- 
cated by the horizontal arrows in the 0.1- to 
10-kilocycle spectra shown in Figures 6 
through 15; the portions of the spectra con- 
tained within these limits give a fair picture of 
the sounds presented to the ear, since the 
change in signal-to-background ratio due to 
use of the filters is quite small for all the 
signals except the port propeller (see the 
fourth column of Table 2). Examination of the 
spectra indicates that the optimal octave band 
was not always selected (see Figures 9 A and 
14A). This circumstance apparently had little 
effect in the first case (line 4 of Table 2) but 
produced a deterioration of 4 decibels in the 
second (line 9). The available data provides 
no information concerning the source of the 
difference in results, but it is interesting to 
note that the deterioration of 4 decibels shown 
in line 9 of Table 2 is very nearly equal to the 


detect with certainty because its character 
was similar to that of the background. If this 
similarity were accentuated by the use of the 
filter, some deterioration would be expected. 
The probability that such an effect occurred is 
indicated by the greater number of errors of 
commission observed when using the band-pass 
filters (see transition curves labeled "effect 
of filters”). The slight improvement shown for 
most of the other cases is of the order of mag- 
nitude of experimental error; if the improve- 
ment is real, however, the effective narrowing 
of the critical bands with increased sensation 
level would be sufficient to account for it. 

Column 6 of Table 2 shows the effect of re- 
moving the low frequencies by inserting a 
1-kilocycle high-pass filter. For those signals 
in which the optimal octave band was below 
1 kilocycle (column three of Table 2), the high- 
pass filter hampered detection; in those cases 
in which the detection frequency was above 
1 kilocycle, the high-pass filter tests gave ex- 
actly the same recognition differentials as were 


c 


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BRITISH TESTS 


87 


obtained without the filter, showing that the 
presence of the lower frequencies did not mask 
the higher. In other words, listening systems 
which do not admit the low frequencies are at 
a disadvantage in detecting machinery sounds. 
This deterioration amounted to 24 decibels in 
the case of the forward planes, whereas in the 
case of the cooling fan, the recorded signal con- 
tained nothing but background noise above 
1 kilocycle. 

Column 5 of Table 2 lists several figures 
showing the relation between signal and back- 
ground at primaudibility when the 1-kilocycle 
high-pass filter was employed. The filtered 
spectra are shown for only those cases in which 
use of the high-pass filter produced a large 
change in RD. It will be observed from these 
figures that a considerable amount of energy 
was passed between 500 and 1,000 cycles; it 
is possible, therefore, that signal recognition 
actually occurred below 1 kilocycle, despite the 
large slope of the spectra in that region. To 
check the latter possibility, it would be neces- 
sary to explore the sounds admitted by the 
high-pass filter with the aid of a narrow band- 
pass filter. 

The near identity of the recognition differ- 
entials obtained without filters and with band- 
pass filters admitting the optimal signal com- 
ponent, and the subjective similarity of the 
primaudible component heard under both con- 
ditions of test, imply that the optimal frequency 
is generally the recognition frequency. Excep- 
tions may be due to (1) inattentiveness, (2) 
elimination of the optimal frequency through 
insertion of filters, as described previously, or 
through inadequacy of system band width, and 
(3) poor response of the detector in the optimal 
region. When the ear is the detector, frequency 
translation may be needed in order to give the 
optimal frequency favorable presentation in 
those cases where it lies beyond the limits of 
the standard reference band. 


Effect of Loudness Level 

These tests were conducted to determine 
whether the recognition differential is signifi- 
cantly affected by the absolute level of the 


background; in other words, by receiver gain. 
The level of the background was set at 50, 70, 
or 90 phons and maintained at one of these set- 
tings while the signals shown in Figures 6 
through 15 were presented to ten observers by 
the random-order method. The transition curves 
obtained are labeled “effect of loudness,” and 
the fractional number of errors of commission 
in the blank presentations is indicated on the 
left-hand borders of the figures. The spectra 
drawn in Figures 6 through 15 may not apply 
equally well to all three loudness conditions, 
since frequency and amplitude distortion are 
difficult to eliminate. It is probable, however, 
that such distortion is not smaller in most 
practical installations than in the system used 
for these tests. 

The 70-phon level was rated most satisfac- 
tory by the observers. The 50-phon setting 
necessitated holding the breath while listening 
and would consequently invite interference 
from room noise at most listening locations in 
the field, whereas the 90-phon setting was con- 
sidered too fatiguing for prolonged listening 
periods. In addition to these general effects, 
some dependence of RD on listening level 
would be expected because of the operation of 
such factors as threshold limitation of the op- 
timal component, improvement in pitch and 
intensity discrimination with increased sensa- 
tion level, and changes in the degree of remote 
as well as adjacent masking (see Figure 5 in 
this chapter and Figure 2 in Chapter 2). The 
effects of these factors depend on the frequency 
of the optimal component and the compositions 
and time patterns of the presented sounds. The 
net result may be either an improvement or a 
deterioration in performance. 

In all but one case (Figure IOC), the ob- 
served effect of loudness level on ease of signal 
detection was fairly small (2 to 3 decibels or 
less). It was found that at the 90-phon setting 
the low-frequency components tended to domi- 
nate the presented sounds, as would be antici- 
pated from the steeper rate of increase in loud- 
ness with sensation level for this region (see 
Figure 5 and Section 2.2.2). This effect prob- 
ably enhanced the listener’s ability to discern 
the low-frequency optimal component indicated 
in Figure 10. Thus, the improvement of 7 


88 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


decibels shown by Figure IOC to result from 
increasing the gain is almost exactly equal to 
the improvement produced by careful listening 
(see Figure lOB and line 3 of Table 1). 

Figure 5 implies that threshold-limited audi- 
bility of optimal components was not a likely 
contingency for the signals shown in Figures 
6 through 15, even at the 50-phon level, except 
for frequencies below about 140 cycles per 
second. The only signal for which such an effect 
might have been expected is shown in Figure 7. 
However, the observed effect of loudness level 
in this case was not significantly greater than 
for most of the other sounds studied. It may 
be noted in this connection that the various 
trials summarized in Figure 9 indicate that the 
low-pitched hum at 100 cycles played no part 
in the detection of this signal under masking- 
limited conditions. Since the octave and critical 
bands centered at 100 cycles have very nearly 
the same width, the 100-cycle component was 
actually fainter relative to background than 
was the 400-cycle hum specified as the primau- 
dible component in Figure 9. Threshold limit- 
ing of the optimal component shown in Figure 
11 is improbable because it rose in pitch during 
the course of the record. 

In two of the ten cases examined (Figures 
8D and 9C) loudness had no effect, either bene- 
ficial or adverse, upon signal detection. In five 
cases (Figures 6D, 7D, IOC, IID, and 14D) 
performance improved progressively with in- 
creased loudness of the mixture, although errors 
of commission showed a parallel increase. 
Although the number of false reports as- 
sumed significant proportions for only one of 
these signals (Figure IOC), the trend implies 
the operation of an unfavorable factor. This 
factor may be increased distortion in the re- 
producing system, and, if so, can probably be 
eliminated by improved design. It is interest- 
ing to note that the optimal component in these 
five cases seems to have occurred below 300 
cycles. 

In one of the remaining three cases (Figure 
12B) the 90-phon level resulted in somewhat 
poorer scores than the 70-phon level; in the 
other two (Figures 13C and 15D) performance 
deteriorated progressively with increasing over- 
all gain. For these three cases, the optimal 


component seems to have occurred at or above 
800 cycles. Although remote masking must be 
considered as a possible explanation of the ad- 
verse effect of increased gain for these three 
signals, the effect should be offset by use of 
high-pass filters, but this did not occur for the 
cases in question (lines 7, 8, and 10 of Table 
2). It seems more likely, therefore, that dis- 
tortions introduced by the test apparatus in- 
fluenced the results. 

Further study is required before detailed 
conclusions can be drawn, but the available 
information appears to warrant several general 
remarks. To begin with, the gain setting 
should be such that the masking background 
is heard at comfortable loudness. Since the 
frequency of the optimal component is gener- 
ally unpredictable, this procedure favors audi- 
bility of the widest possible band of frequencies 
without undue risk of operator fatigue or dis- 
tortions produced by overload of the listening 
system. Because of the unpredictable fre- 
quency of the optimal component during search, 
no simple selectivity scheme is available for 
emphasizing or improving the presentation of 
a narrow detection band ; all the components in 
the presentation band contribute to the loud- 
ness and thereby limit the sensation level 
which may be obtained within the detection 
band. In the second place, auditory acuity 
tends to improve with increasing sensation 
level even in the presence of masking. 

4.1.8 Effect of Simulated Hydrophone 
Directivity 

The purpose of these tests was to investigate 
the improvement in audibility to be expected 
from use of directional hydrophones capable 
of discriminating against nondirectional am- 
bient-noise backgrounds. Since such hydro- 
phones also give some discrimination against 
the high-frequency components in the target 
sounds, two filters were used to simulate the 
directional condition: (1) a background filter 
(with the discrimination characteristic shown 
in Figure 24) which was inserted between the 
pre-amplifier of the ambient-noise sound head 
and the mixer and (2) a signal filter (with the 


■RKSTRICTEdJj 


BRITISH TESTS 


89 


discrimination characteristic shown in Figure 
24) which was inserted at the corresponding 
point in the signal channel. Figure 25 repre- 
sents the difference between the curves in 
Figure 24 and thus indicates the improvement 
in signal-to-background ratio at various fre- 
quencies which is afforded by the directional 
properties of the particular hydrophone, an 
8-foot ring, used as a model for these tests. 


refer to sound levels at the inputs to the filters, 
that is, they correspond to sound-in-the-water. 
The pertinent observations are summarized in 
Table 3 and are discussed below. 

Column 3 lists the estimated frequencies of 
the signal components detected in the direc- 
tional and nondirectional conditions of test. 
This estimate has been made by noting the 
optimal frequencies, that is, the frequencies 



Figure 24. Effect of signal and background 
filters. The response of the latter was designed 
to simulate the discrimination against nondirec- 
tional ambient of an 8-foot ring hydrophone. 


The discrimination shown in Figure 24 applies 
only to conditions in which the target acts as 
an extended source. Since the signals used in 
these tests represent quiet submarines, detect- 
able only at fairly short range, the design of 
the signal filter is appropriate. 

In the directional condition, the reproducing 
system simulated the sounds which would be 
received in a highly directional hydrophone 
trained on-target, and not subject to control 
by the operator; in the nondirectional condi- 
tion, the system simulated the sounds which 
would be received from the same target, in the 
same prevailing background, through a non- 
directional hydrophone. The sounds were pre- 
sented by the increasing level method, and at 
a level of 70 phons, to five reliable observers, 
four of whom had participated in all the tests 
previously described. The spectra of these 
sounds, as presented in each condition of test, 
are shown in Figures 16 through 23. The re- 
sulting transition curves, which are given in 
the same figures, represent the average of two 
determinations for each condition ; and the 
horizontal shift between the curves marked 
“with'' and “without" filters shows the im- 
provement in audibility, or change in RD. The 
recognition differentials, as well as the levels of 
overall background indicated on the ordinates. 





























N 
















\ 


















I 2 4681 2 4681 2 4681 

10 100 1000 10,000 
FREQUENCY IN CYCLES 

Figure 25. Improvement in signal-to-water noise 
ratio produced by filters. 

corresponding to the distance of closest ap- 
proach in the spectra shown in Figures 16 
through 23. In column 4 are tabulated the 
spacings between signal and background spec- 
tra at the optimal frequencies. It will be ob- 
served that, while this spacing may be as small 
as 0 decibel, it averaged about 6 to 7 decibels, 
in agreement with the discussion in Section 
4.2.3. In column 5 are shown the various 
amounts of improvement in detectability of the 
signals due to simulated hydrophone directiv- 
ity. These improvements have been read from 
the transition curves, and, for comparison with 
them, the sixth column indicates the computed 
improvement. 

When hydrophone directivity is introduced, 
the shapes of the spectra representing the 
sounds-at-the-ear are changed, as shown in 
Figures 16 through 23. Thus, two types of 
cases must be distinguished when computing 
the improvement in detectability resulting 
from directivity: (1) those in which the same 
signal component is recognized in both condi- 
tions of test and (2) those in which a different 
signal component becomes primaudible in the 
directional and nondirectional conditions. Ex- 
amination of the spectra indicates that the de- 
tection frequency was not changed by the simu- 
lated directivity in six of the eight cases 
studied but was changed in the remaining two 


RKSTRICTED 


90 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


(see the fifth and seventh signals in Table 3 
and also the corresponding spectra). The cases 
in which the frequency of the optimal compo- 
nent varied are discussed separately from the 
others. 


essential parallelism of the signal and back- 
gi'ound spectra at the higher frequencies in 
Figures 16 through 18. Figure 25 shows that 
the maximum objective improvement, occur- 
ring at about 3 kilocycles, amounted to 13 


Table 3. Effect of hj'drophone directivity. 


Signal 

Transmitted 
spectra at 
primaudi- 
bility 

Estimated 
detection 
frequency 
in cycles 

Spacing be- 
tween spectra 
at optimal fre- 
quency in db 

Observed 
improvement 
of RD 
in db 

Computed 
improvement 
of RD 
in db 

1. Propellers at 200 rpm 

Figure 16A 

Broad band 
above 2 kc 

6 

11 

13 


Figure 16B* 

Broad band 
above 2 kc 

6 



2. Prop)eller at 200 rpm 

Figure 17A 

Broad band 
above 1 kc 

6 

11 

13 


Figure 17B* 

Broad band 
above 1 kc 

7 



3. Motors and prop>ellers at 160 rpm 

Figure 18A 

Broad band 
above 1 kc 

10 

11 

13 


Figure 18B* 

Broad band 
above 1 kc 

10 



4. Gear noise at 90 rpm 

Figure 19A 

800 

8 

6.5 

8 


Figure 19B* 

800 

7 



5. Gear noise at 60 to 70 rpm 

Figure 20A 

200 

7 

3.5 

3 

Figure 20B* 

800 

5 



6. Ballast pump 

Figure 21A 

200 

6 

3.5 

2 


Figure 21B* 

200 

7 



7. Circulating pumps at full speed 

Figure 22A 

150 

0 

4 

5 


Figure 22B* 

1,100 

0 



8. Circulating pumps at half sp>eed 

Figure 23A 

540 

7 

6.5 

5.5 


Figure 23B* 

540 

8 




*Filters were inserted to simulate the effects of h\'drophone directivity. 


As explained in the following paragraph, 
when the detection frequency is the same for 
both conditions of test, the improvement in RD 
should be numerically equal to the objective 
improvement in signal-to-background ratio af- 
forded at the optimal frequency by the directiv- 
ity filters. The various amounts of objective 
improvement for all signals other than 5 and 7 
have been computed from column 3 and Figure 
25 and are entered in column 6. In determining 
the computed improvement from Figure 25, the 
maximum possible improvement was selected 
in each instance, on the assumption that the 
signal would become primaudible as soon as the 
required detection ratio was attained in the 
optimal frequency band. Thus, lines 1, 2, and 
3 represent situations in which the signals 
probably became primaudible within a fairly 
wide band of frequencies, as indicated by the 


decibels, but that a possible improvement equal 
to or greater than 10 decibels characterized 
the entire band between 1 and 6 kilocycles. The 
presumptive wide-band quality of the primau- 
dible component in these cases probably ac- 
counts, in part, for the fact that the observed 
improvement fell somewhat short of the com- 
puted maximum improvement, which is as- 
sociated with a narrower frequency band. Simi- 
larly, two distinct signal components, at 0.3 
and above 1.0 kilocycle in Figure 18A, and at 
0.3 and 0.8 kilocycle in Figure 19A could have 
and possibly did become primaudible in the 
nondirectional condition ; here again, the maxi- 
mum possible objective improvement (associ- 
ated with the higher frequency) has been en- 
tered in column seven for the reason stated 
above. 

The procedure outlined for computing the 


f^STRICTED ^ 


BRITISH TESTS 


91 


improvement in RD may appear curious at first 
glance, since the recognition differentials give 
the signal-to-background ratio in terms of the 
overall intensities of the sounds, whereas the 
objective improvements (as read from column 
3 and Figure 25) apply to specific frequency 
regions. This procedure is not contradictory, 
however, since the directivity filter diminishes 
the level of the masking background at the 
optimal frequency relative to the overall level 
of the unfiltered background, which serves as 
the reference standard for both conditions of 
test. When the intensity of the optimal signal 
component is reduced a corresponding amount 
during a listening test, it remains primaudible, 
but all the other components in the signal are 
reduced comparably. Thus, the RD (or overall 
signal-to-background ratio at primaudibility) 
in the directional condition will be less than 
that in the nondirectional condition by an 
amount equal to the objective improvement 
in signal-to-noise ratio at the recognition fre- 
quency. 

This improvement is given by Figure 25 and 
includes the effect of the directivity filter in 
the signal channel. The overall signal-to-over- 
all noise ratio given by this calculation is that 
at the input to the filters (and corresponds to 
the sound-in-the-water, or RD, rather than 
that at the output, or presentation differential) . 
In this particular case, the two types of differ- 
entials would differ by a nearly constant factor 
amounting to 3 to 4 decibels. This circumstance 
arises from the fact that the bulk of the energy, 
in sounds with large negative spectrum slope, 
is concentrated in the lower frequencies, where- 
as the directivity filters affect primarily the 
upper frequencies. The first part of the pre- 
ceding statement may be demonstrated by re- 
plotting the filtered and also the unfiltered 
background spectra depicted in Figures 16 
through 23 on scales linear with respect to 
energy and frequency. The ordinates will then 
represent energy per cycle and the abscissas, 
cycles. The integrated areas under each of the 
curves, obtained by counting squares or by 
planimetry, will represent the overall energies 
of the filtered and unfiltered sounds. The dif- 
ference between them is of the order of 4 
decibels for the masking background, as also 


demonstrated by sound level measurements 
during the tests. The overall levels of the 
signals, on the other hand, were hardly modi- 
fied by the directivity filter in the signal chan- 
nel, since that filter affected the highest fre- 
quencies only, and these contained a negligible 
fraction of the total energy. 

The agreement between observed and com- 
puted changes in RD shown in columns 5 and 
6 serves to verify two points emphasized in the 
preceding and ensuing discussions: (1) recog- 
nition in the case of broad-band sounds de- 
pends, in general, not on the total energies, 
but on the signal-to-noise ratio in a narrow 
frequency band; and (2) the optimal frequency 
is usually the recognition frequency. This re- 
emphasis is, in fact, one of the chief reasons 
for computing column six. Another reason 
is the desirability of assessing the factors 
which determine the practical effects of hydro- 
phone directivity. As indicated in Section 4.1.3, 
the numerical values of wide-band recognition 
differentials (and hence, their calculation) 
have little intrinsic interest. Indeed, the avail- 
able information tends to support the view that 
ultimately operational prediction and analysis 
may be performed most satisfactorily by direct 
comparison of signal and background spectra, 
without explicit consideration of overall levels 
(see Section 4.2.2). 

It is worth noting at this point that the cri- 
terion adopted for predicting the improvements 
in recognition differentials will be valid only 
if the change in the shapes of the spectra does 
not alter the signal-to-noise ratio required for 
primaudibility in the detection band. Such 
changes in the detection ratio might be pro- 
duced by the modified sensation level at which 
the primaudible component is identified, or by 
differences in the degree of remote masking, 
of time pattern, or of distortion introduced by 
the test apparatus. In practice, hydrophone 
directivity may also change the peakiness of 
the received background and thereby the sig- 
nal strength needed for primaudibility (see 
Figure 3 in Chapter 5). As indicated by col- 
umn 4 in Table 3, the detection ratio for a 
given signal was essentially independent of the 
simulated directivity. This observation appears 
quite reasonable for the cases in which the 


t 


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92 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


same signal component was detected in each 
condition of test but is probably sheer accident 
for the cases in which different components 
were identified in the directional and nondirec- 
tional tests. 

In the case of lines 5 and 7, the improve- 
ment also depends on the effectiveness of the 
directivity filters in reducing the level of back- 
ground but the degree of improvement is modi- 
fied by the fact that the frequency and charac- 
ter of the primaudible component are variable 
rather than fixed. Thus, for the fifth signal, 
the optimal component occurs at 200 cycles in 
the nondirectional condition and at 800 cycles 
in the directional. These results are in agree- 
ment with the approximate prediction of theory 
(see Section 4.2.2). Examination of Figure 
20A shows that, because of the relative shapes 
and positions of the spectra, the spacing be- 
tween nondirectional background and the (un- 
detected) component at 800 cycles amounted to 
10 decibels. In the second condition of test 
(Figure 20B) the spacing to the (detected) 
800-cycle component is reduced to 5 decibels; 
in other words, this component, and therefore 
the entire signal, must be raised by 5 decibels 
to render it detectable, thereby losing 5 of the 
potential 8-decibel improvement shown by Fig- 
ure 25 to accrue from directivity for a signal 
component primaudible at 800 cycles. The net 
gain, 3 decibels, has been entered in column 6. 
Thus, the improvement is greater than would 
have been observed had the 200-cycle compo- 
nent been detected in both conditions, but less 
than would be obtained for a fixed 800-cycle 
component. While the corresponding gain in 
range for such cases will in general have an 
intermediate value, as illustrated here, bearing 
accuracy will be improved in proportion to the 
change in frequency of the optimal component. 
The computed improvement for the seventh 
signal was found by this same method. 

Several conclusions are worth noting at this 
point. From the last two columns in Table 3, 
it follows that knowledge of the signal and 
background spectra, together with the discrimi- 
nation of the hydrophone, makes it possible 
to predict improvement in performance to 
within about 2 decibels. As a corollary, it fol- 
lows that the hydrophone whose discrimination 


is shown in Figure 24 should give better gen- 
eral performance than the line hydrophone with 
the characteristic illustrated in Figure 18 of 
Chapter 3, provided the discrimination of hy- 
drophones against nondirectional ambient noise 
is also an adequate index of their discrimina- 
tion against self-noise. From the spacings 
shown in column four, it may be concluded that 
increasing the slope of the background spec- 
trum from 6 decibels per octave (nondirec- 
tional condition) to 10 decibels per octave (di- 
rectional condition) has no adverse effect on 
signal audibility. In fact, the loudness contri- 
butions from the various components in the 
directional background spectrum are more 
nearly equal, in the band between 0.2 and 6 
kilocycles, than is the case for the nondirec- 
tional background (see Figures 16 through 23) . 
Protracted listening to the directional back- 
ground therefore is probably less fatiguing, and 
search for the optimal component may be 
somewhat easier, since the various frequencies 
receive more nearly equal emphasis at the ear 
and the high frequencies are less annoying. To 
assure effective listening over the widest usable 
frequency band, the outputs of practical sys- 
tems should probably tend to parallel the shape 
of the audibility threshold. It will be observed 
that the relation between the 70-phon back- 
ground spectrum and the estimated thresholds 
for distributed sounds is somewhat different 
for the directional and nondirectional cases be- 
cause of the changed composition of the pre- 
sented sound. Finally, the improvement pro- 
duced by the directional condition is greater 
for the propeller cavitation sounds in Table 3 
than for the machinery sounds since cavitation 
was detected at the higher frequencies, where 
the simulated discrimination was greatest. 


42 UCDWR TESTS 

A thorough and comprehensive program of 
listening tests, in progress at the time of writ- 
ing at the San Diego Laboratory of the Uni- 
versity of California Division of War Research 
[UCDWR] was undertaken to obtain funda- 
mental information about the auditory detec- 
tion of underwater sounds. The methods and 


KSIRK/ITia 


UCDWR TESTS 


93 


results of this study, some of which have been 
made available in private communications prior 
to publication and some of which have appeared 
in published reports,®-® are described in Sec- 
tions 4.2.1 through 4.2.6. 

Techniques 

The apparatus used in these tests is shown 
schematically in Figure 1. Care was taken to 
provide practically distortionless gain in the 
amplifier components. The recorded sounds 
which were used are identified in the various 
figures and tables presented below. The selec- 
tions studied were obtained with a variety of 
sonic and supersonic receivers and include sig- 
nals from ships, submarines, and torpedoes ; in 
a few cases, simulated propeller sounds were 
employed. The sonic backgrounds included am- 
bient noise, with and without shrimp crackle, 
and also simulated deep-sea ambient noise; the 
supersonic backgrounds were self-noise and 
ambient noise, with and without shrimp 
crackle. The simulated ambient was obtained 
from a broad-band, thermal noise source with 
a spectrum slope of —6 decibels per octave; 
simulated propeller sounds were obtained by 
similar means and were given various rates 
and degrees of amplitude modulation. Combi- 
nations of signal and background pairs from 
this library of recordings were presented 
through headphones worn by the observers. In 
general, the properties of the receivers and the 
techniques of recording and playback were 
such that a moderately faithful replica of the 
sounds-in-the-water was presented to the head- 
phones. Since not all the available recordings 
were of high quality (their merit is indicated 
in the various tables, the rating being based on 
quality in a 0.1- to 10-kilocycle band and on 
freedom from extraneous noise) and since not 
all signal and noise pairs were obtained with 
the same hydrophone, the observations are not 
equally typical of the field situation. Thus, the 
peaks at 2 and 5 kilocycles in the spectrum of 
the recorded shrimp crackle (see Figure 28) 
are artifacts introduced by resonances in the 
hydrophone used for recording. More recently, 
higher quality recordings have been obtained 
by transmitting the signals over an FM link 


from the receiving vessel to a shore station, 
where the recording can be done under better 
conditions than are usually available on board 
ship. Also, apparatus for monitoring ship 
sounds has been installed at the entrance to 
San Diego harbor. The hydrophones used in 
this installation are mounted on the harbor 
bottom; their outputs are transmitted by cable 
to a shore station, where a frequency trans- 
lator makes it possible to bring 8-kilocycle 
bands of supersonic frequencies into the audio 
region. Thus, high quality recordings of ship 
sounds, in the interval from 0.1 to 30 kilo- 
cycles, can be obtained. 

The five observers who participated in each 
test were either members of the laboratory 
staff or enlisted men, newly graduated from 
the United States Navy West Coast Sound 
School. Since no individual in the latter group 
was available for more than a week, at least 
one experienced observer from the laboratory 
personnel was included in each test in order 
to serve as a control. The weekly addition of 
new observers tends to make the results ob- 
tained more representative and permitted study 
of such factors as learning and the effects of 
selection and Sound School training. 

Various presentation methods were studied. 
Only the results obtained by means of random- 
order presentations are given in the tables and 
figures, the effect of other presentation meth- 
ods being outlined briefly in Section 4.2.6. 
Random-order tests lasted between 20 and 30 
minutes and were begun by permitting the 
listeners to familiarize themselves with the 
signals and backgrounds which were to be 
mixed in the subsequent test. The background 
was maintained at a constant level correspond- 
ing to between 60 and 75 phons, the gain ap- 
plied to the mixture being subject to some con- 
trol by the individual listeners, and the mix- 
tures were presented at 10-second intervals for 
periods of 5 seconds. Signal level was varied 
in steps of 2 decibels and randomizing was 
achieved by means of an automatic selector. 
This selector controls the gain setting in the 
signal channel according to a predetermined 
pattern which is punched upon a moving tape. 
Several tapes were used in order to eliminate 
learning of the presentation order. If an ob- 


94 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


server reported that the signal was audible in 
more than one of the blank presentations, the 
entire test for that individual was rejected. 

A recently developed device for recording 
listener response consists of a sound range 
recorder modified to give a group of parallel 
traces, which show at a glance the gain set- 
ting in the signal channel at any time during 
a test and the simultaneous response of the 
observers. The latter indicate their judgments 
by throwing a hand switch from a neutral 
position to either of two positions correspond- 
ing to “audible’’ and “inaudible.” This pro- 
cedure saves time and confusion in recording 
judgments and scoring performance; it indi- 
cates indecision and time needed to reach a de- 
cision; and it gives a compact and permanent 
record of simultaneous signal levels and re- 
sponses. The latter feature permits elimination 
of the unrealistic subdivision of the test into 
a group of discrete listening intervals each of 
which contains either a signal or a blank. This 
subdivision is necessary in the usual test pro- 
cedure in order to allow time for writing down 
responses and also to simplify the process of 
scoring the test, which involves setting up a 
correspondence between items in the response 
series and in the signal level series. Thus, it 
is relatively simple by means of the procedure 
just outlined to score performance during long 
intervals of continuous listening designed to 
simulate the shipboard situation. 

“Following every test^ each listener’s de- 
scription of the character of the primaudible 
signal was recorded. The same signal was pre- 
sented to all five listeners but usually each 
described it in different words. Also the oc- 
tave or half-octave band, necessary to pass the 
frequency component for making the signal 
primaudible, was determined by listening while 
a variable set of half-octave filters was inserted 
into the signal channel. It was generally found 
that the primaudible components of the signal 
were located in a limited frequency band less 
than an octave in width. In the usual case, 
where it was apparent from the listeners’ com- 
ments that all had detected the same component 
of the signal, it was sufficient to determine the 
primaudible frequency only once.” Care was 
taken, in obtaining these comments, to have 


the listeners report the character of the signal 
at primaudibility, and not the more striking 
feature of the sound when the signal was sev- 
eral decibels above primaudibility (see also 
Section 4.1.5). Occasionally, the primaudible 
component extended over a relatively wide 
band (see Figures 38 and 55). 

The overall levels in a 0.1- to 10-kilocycle 
band of all the sounds used were measured, 
and the spectra were obtained by means of a 
50-cycle band-pass filter. Therefore tonal com- 
ponents appear in the spectra as peaks 50 
cycles wide (see also Figures 59 and 60). 
Power was measured with a level recorder of 
high writing speed so that time patterns could 
be studied. Time patterns for a number of 
typical cases are illustrated in many figures in 
this chapter. The power level traces represent 
independent measurements, within the recog- 
nition band, made in the noise and signal chan- 
nels when the gain settings were adjusted to 
the ratio which rendered the signal primau- 
dible. The time-intensity-frequency traces, de- 
scribed in Section 3.1.1, were made with a 
45-cycle filter and with the aid of a tilting net- 
work which increased the gain of the higher 
frequencies at the rate of 6 decibels per oc- 
tave, in order to confine the effective range of 
transmitted power levels to the contrast range 
of the recording paper. The rms spectra in 
the figures represent the ratio of power in a 
50-cycle band to the overall power in the stand- 
ard reference band and are plotted as a func- 
tion of the midfrequencies of the 50-cycle 
bands. Such spectra will be termed 50-cycle 
spectra; in the range 0.1 to 1.5 kilocycles they 
correspond very closely to critical-hand spectra 
(power measured in frequency intervals of 
width equal to a critical band; see Figures 76 
through 79). At the higher frequencies, how- 
ever, the 50-cycle spectra differ considerably 
from critical-band spectra, and this must be 
borne in mind when interpreting the observa- 
tions. In order to investigate the presence of 
tonal components, comparisons were made of 
the power passed by half-octave and 50-cycle 
filters in the frequency region where such com- 
ponents appeared to exist. The significance of 
the arrows and the shaded areas in these dia- 
grams is explained in the next section. 


UCDWR TESTS 


95 


Observed Recognition Differentials 

The results obtained for sonic target sounds 
are given in Table 4, which is subdivided into 
three sections, according to the background 
used in the tests. The relation between signal 
and noise spectra at primaudibility is shown 
for some typical cases in Figures 26 through 
53, and the legends in these figures refer to 
the corresponding data in Table 4. These 
primaudibility charts, as well as the others in 
this volume, give, not the sums of signal and 
background levels, but independent spectra for 
each of the sounds, referred to the same over- 
all level. Hence signal-to-noise ratios may be 
obtained by inspection. As pointed out before, 
it was found that a wide-band acoustic signal 
generally becomes primaudible within a re- 
stricted frequency interval. These recognition 
frequencies are indicated in columns five and 
ten of Table 4. Alternatively, they may be read 
directly from the spectra by noting the fre- 
quency at which signal and noise curves are 
closest to each other (see, for example, the 
component at 360 cycles in Figure 26) . 

This close approach of signal and noise spec- 
tra at primaudibility implies that the critical- 
band criterion may properly be extended to 
complex sounds of the kind illustrated, since 
that criterion requires that signal and noise 
energies approach unity in at least one critical 
band. That the various critical bands are, to a 
first approximation, independent of each other 
is indicated by the fact that simultaneous, equal 
stimulation of two critical bands does not nec- 
essarily improve recognition (see Figure 34). 
There are some indications, however, that si- 
multaneous stimulation of multiple critical 
bands may affect recognition (for example, 
see Section 4.2.3). 

It was observed, when the components in the 
optimal 50-cycle band displayed a strong 
rhythmic fluctuation, that the average value 
of the rms signal level within that 50-cycle 
band could be as much as 12 decibels below 
the rms noise level at primaudibility (see lines 
26 and 33 of Table 4, and Figures 40, 41, and 
42). In all such cases, however, fairly consis- 
tent agreement with the critical-band criterion 
was obtained when the signal-to-noise ratio in 


the optimal critical band was computed from 
the recurrent peak values of the rms signal 
level. This is illustrated (1) in columns 8 and 
9 of Table 4; (2) in the primaudibility charts 
in Figures 26 through 53, where the arrows 
and shaded areas signify fluctuating tones and 
fluctuating frequency bands, respectively; and 
(3) in the power level traces (included among 
Figures 26 through 53) which show the tem- 
poral relation between signal and noise levels 
in the optimal 50-cycle band at primaudibility. 
It will be noted from Table 4, however, that 
even the recurrent peak level of the signal may 
fall 2 to 3 decibels below the mean noise level, 
both measured in the same critical band. This 
may imply that fluctuating sounds are some- 
what more audible than steady ones, due to 
the effects of auditory motion (see Sections 
5.1 and 5.6), or that the measured level of a 
rapidly fluctuating component cannot build up 
to full strength when restrictive filters are used 
(see Section 4.2.3) . These points may be worth 
investigating for the cases of frequency- and 
amplitude-modulated tones and may have some 
bearing on noise reduction measurements as 
well as on the choice of proper operating con- 
ditions for a vessel engaged in evasive ma- 
neuvers. 

It follows, therefore, that signal-to-noise ra- 
tios at primaudibility may be predicted directly 
from a knowledge of the spectra involved, pro- 
vided the frequency analyses are made with 
adequate resolution (that is, analyzer band 
width not exceeding critical band width) and 
provided also the dynamic response of the re- 
corder approximates that of the ear. Since the 
latter has a build-up time of between 0.1 and 
0.2 second, it can react rapidly enough to re- 
spond to the maximum levels reached by a 
pulsating signal modulated at 10 cycles or less. 

To test the effects of recorder speed, traces 
were made using a rapid writing speed of 400 
decibels per second and a slow speed of 100 
decibels per second. In measurements of ran- 
dom noise, such as deep-sea ambient or shrimp 
noise, the average level recorded was practi- 
cally independent of writing speed. When mea- 
surements are made of rapidly modulated sig- 
nals like those illustrated in Figures 39 and 
45, the rhythmic peak levels are slightly closer 




ESTRICTEi 


96 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


Table 4, Primaudibility of sonic sounds. 





Character 

of 

primaudible 

signal 

Recog- 

Signal-to-noise 
ratio in db in a 
0.1-10-kc band 

Signal-to-noise 
ratio in db in the 
optimal 50-cycle 
band at 
primaudibility 

Midfre 
quency 
in cycles 
of opti- 
mal 50- 

Signal 

Figure 

Merit 

of 

signal 

nition 
frequency 
region in 

80 

per cent 

50 

per cent 



cycles 

recog- 

nition 

prob- 

ability 

recog- 

nition 

prob- 

ability 

Aver- 

age 

ratio 

Ratio for 
rhythm 
peaks 

cycle 

band 


A. Background: simidated deep-sea noise. 


1. Anchored S- class 
submarine, charging 
batteries with diesels 

36 

Good 

Low-frequency 

rhythm 

140-280 

- 4 

- 6 

-3 


120 

2. Anchored S-class sub- 
marine, charging bat- 
teries with diesels 


Fair 

Low-frequency 

rumble 


- 8 

-10 

0 


400 

3. S-class submarine at 

5 knots and periscope 
depth 


Fair 

Rhythmic swish 


-10 

-12 

-2 


900 

4. Submerged S-class 
submarine at 6 knots 


Fair 

Rhythmic churn- 
ing 


- 9 

-11 

-2 


520 

5. S-class submarine at 

3 knots and periscope 
depth 


Good 

Rhythmic chug 

200-400 

- 8 

- 9 

-2 


250 

6. Submerged subma- 
rine at 5 knots 


Fair 

Rhythmic churn- 
ing 

.... 

-11 

-13 

-5 

0 

450 

7. Fleet-type submarine 

29 

Good 

Squeaking 

Above 3 , 200 

-12 

-14 


8 

7,000 

at 5 knots and peri- 
scope depth 


Good 

Rhythmic hiss 

Above 3 , 200 

-12 

-14 

-7 

0 

7,000 

8. Fleet-type submarine 
at 60 rpm (3 knots) 
and periscope depth 

26 

Good 

Hum or whine 

280-400 

- 9 

-12 

0 


360 

9. Surfaced fleet-type 

34 

Good 

Tinny grinding 

1,130-1,600 

— 4 

- 6 

-1 


1,400 

submarine underway 
at 12 knots 


Good 

Low rumble 

140-280 

- 4 

- 6 

-2 


120 

10. German Type VII 

C submarine (GRAF) 
at 23 ^ knots and peri- 
scope depth 

50 

Fair 

Low bubbling 
rhythm 

200-400 

- 9 

-11 

-8 

2 

300 

11. Convoy (13 mer- 
chant vessels, 6 large 
tank lighters, and 3 
PC escorts) 

53 

Fair 

Hum or whine 


-10 

-12 

-3 

0 

390 

12. Tanker, 1 mile away 

44 

Fair 

Low rhythmic 
noise, whine 

565-800 

- 7 

- 9 

0 


680 

13. Freighter, 7,000 

tons. 442 feet long, at 
10 knots 

38 

Good 

Rhythmic swish 
and slow 
rhythm 

Above 4 , 500 

- 8 

-12 

-1 

1 

8,600 

14. Destroyer, 2,870 

tons, 381 feet long, at 
15 knots 

43 

Good 

Low tone with 
propeller 
rhythm 


- 9 

-11 

-2 


250 

15. Torpedo (3-turbine 
type), at 45 knots 

47 

Good 

High-pitched 
whine or hum 

800-1 , 130 

- 9 

-11 

6 


970 

16. Torpedo (electric 

type), at 30 knots 

49 

Good 

Hum and whine 


-15 

-17 

-1 


600 



UCDWR TESTS 


97 


Signal 


Figure 


IMerit 

of 

signal 


Table 4. {Continued) 


Character 

of 

primaudible 

signal 


Recog- 
nition 
frequency 
region in 
cycles 


Signal-to-noise 
ratio in db in a 
0.1-10 kc band 

80 

50 

per cent 

per cent 

recog- 

recog- 

nition 

nition 

prob- 

prob- 

ability 

ability 


Signal-to-noise 
ratio in db in the 
optimal 50-cycle 
band at 
primaudibility 


Aver- 

age 

ratio 


Ratio for 
rhythm 
peaks 


Midfre- 
quency 
in cycles 
of opti- 
mal 50- 
cycle 
band 


B. Background: shrimp crackle. Merit: very good. 


17. Anchored S-class 

submarine, charging 
batteries with diesels 

37 

Good 

Low-frequency 

rhythm 

140-200 

- 8 

-10 

4 


150 

18. Anchored S-class 
submarine, charging 
batteries with diesels 


Fair 

Rhythmic rum- 
ble 

280-565 

-18 

-20 

-2 


400 

19. Submerged S-class 
submarine at 6 knots 


Fail- 

Rhythmic churn- 
ing 

400-800 

-16 

-18 

-2 

0 

550 

20. Submerged S-class 
submarine at 5 knots 


Fair 

Rhythmic churn- 
ing, whine 

400-800 

-11 

-13 

2 


540 

21. S-class submarine at 

3 knots and periscope 
depth 


Good 

Rumble 

200-280 

-11 

-15 

-1 


250 

22. Fleet-type subma- 
rine at 5 knots and per- 
iscope depth 

32 

Good 

Whine 

400-565 

-13 

-15 

0 


500 

23. Fleet-type subma- 
rine at 60 rpm (3 
knots) and periscope 
depth 

28 

Good 

Hum or whirr 

280-400 

-19 

-21 

0 


360 

24. Surfaced fleet-type 

35 

Good 

Low rumble 

140-280 

- 4 

- 7 

6 


120 

submarine underway 
at 12 knots 


Good 

Whin- 

1,130-1,600 

- 4 

- 7 

-3 


1,400 

25. Convoy (13 mer- 
chant vessels, 6 large 
tank lighters, 3 PC 
escorts) 


Fair 

Rhythmic hum 

280-565 

-20 

-21 

-3 

0 

390 

26. Freighter, 7,000 

tons, 442 feet long, at 
10 knots 

40 

Good 

Rhythmic chug 

Below 400 

-12 

-14 

-6 

-1 

260 

27. Torpedo (3-turbine 
type), at 45 knots 


Good 

Whine 

800-1 , 130 

-19 

-21 

0 


950 


C. Background: water noise recorded at a depth of 300 feet; water depth 1,000 feet. 


28. S-class submarine, 
at 6 knots and peri- 
scope depth 


Fair 

Rhythmic chug- 
ging 


- 9 

-11 

- 5 

-2 

500 

29. Submerged S-class 
submarine at 5 knots 


Fair 

Pulsating hum 


-11 

-13 

0 


. 360 

30. S-class submarine at 

3 knots and periscope 
depth 


Good 

Low rumble 


- 8 

-10 

2 


250 

31. Convoy (13 mer- 
chant vessels, 6 large 
tank lighters, 3 PC 
escorts) 


Fair 

Rhythmic churn- 
ing 


-10 

-12 

-5 

-2 

390 

32. Tanker, 1 mile away 


Fair 

Rhythmic chug- 
ging 


- 9 

-11 

4 


190 

33. Freighter, 7,000 

tons, 442 feet long, at 
10 knots 


Good 

Rhythmic churn- 
ing 


-18 

-20 

-12 

-3 

180 

34. Torpedo (3-turbine 
type), at 45 knots 


Good 

Tone 


-10 

-12 

0 


200 

35. Torpedo (3-turbine 
type), at 45 knots 


Good 

Whine 


-10 

-12 

-4 


950 


98 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


to the noise levels in the optimal critical bands 
when a fast writing speed is used. With slowly 
modulated signals, however, virtually any writ- 
ing speed is satisfactory. 

The following is a summary of the results 
listed in Table 4. 



Signal-to- 
noise ratio 
in db in 
0.1-10 kc 
band for a 
50 per cent 
recognition 
probability 

Average 
signal-to- 
noise ratio 
in db in 
50-cycle 
band at 
primaudi- 
bility 

Signal-to 
noise ratio- 
in db in 
50-cycle 
band for 
rhythm 
peaks at 
primaudi- 
bility 

Mean (35 measure- 

-12.9 

-1.5 

0.0 

ments) 




Average deviation 

3.1 

2.7 

1.6 

from mean 





It is evident that the ratio of maximum sig- 
nal-to-average noise, both measured in the op- 
timal critical band, gives the most consistent 



4681 2 4681 2 4681 

100 1000 10,000 
FREQUENCY IN CYCLES 

Figure 26. Audibility of sonic noise from a fleet- 
type submarine at 3 knots and periscope depth, 
masked by simulated deep-sea noise. RD for 
presentation band shown: — 12 decibels. Char- 
acter of primaudible signal : hum or whine. 
Primaudible frequency: 360 cycles. See Table 4, 
line 8. 

prediction of the condition for primaudibility. 
The mean value of —1.5 decibels obtained when 
using the average signal level in the optimal 
50-cycle band is probably not a reliable index 
of the amount of modulation which is typical 
of ship and submarine signals. Only a few of 
the primaudible components studied were am- 


plitude modulated, and some of these fluctuated 
by as much as 15 to 20 decibels. Similarly, the 
comparatively small average deviation of 3.1 
decibels among the wide-band recognition dif- 
ferentials is not necessarily typical, since the 
magnitude of the scatter depends on the degree 
of similarity of signal and noise spectra, on 
time patterns, and phase and harmonic rela- 
tions. Consequently, no universal index, such 
as the recognition differential in terms of over- 
all power, can have useful predictive reliabil- 
ity. 

An analyzer band width of 50 cycles was 
employed through the range 0.1 to 10 kilo- 
cycles. This approximates the critical band 
widths in the region below about 1.5 kilocycles 
(see Figure 17 in Chapter 2) but is narrower 
than the critical bands for the higher frequen- 
cies. The 50-cycle band width was generally 
satisfactory, however, because few of the 
sounds studied contained prominent tones 
above 1 to 2 kilocycles. Strictly speaking, a 
critical-band analysis can be performed only 
with a group of discrete Alters of flxed mid- 
frequency and band width. This is compli- 
cated, experimentally. It was found satisfac- 
tory to use instead a heterodyne analyzer with 
a 50-cycle band width of variable midfrequency. 
Obviously, the error introduced by failure to 
use the correct critical band width will be the 
same for signal and background when these are 
distributed sounds with approximately the 
same slope in the region measured. Hence, 
this error cancels out when the difference be- 
tween signal and noise levels is computed. Spe- 
cific correction to the proper band width must 
be made, however, when tonal components are 
present above 1.5 kilocycles. In the case of 
distributed sounds, the 50-cycle spectra are 
uniformly 17 decibels higher in level than the 
1-cycle spectra (10 log 50 — 17 decibels) and 
have the same slope as the latter (see also 
Figures 59 and 60). 

Figure 29 represents a situation in which 
the signal was recognized as a high-pitched 
squeak, primaudible at 7 kilocycles. The opti- 
mal signal-to-noise ratio at primaudibility, in 
this case, was 4-8 decibels when signal and 
noise were measured with the 50-cycle filter 
(see line 7 in Table 4) . Since there is a promi- 


UCDWR TESTS 


99 



Figure 27. Time-frequency-intensity analysis for sonic noise from a 3-knot submarine (Figure 26). 



4681 2 4681 2 4681 

K>0 1000 10,000 

FREQUENCY IN CYCLES 



4681 2 4681 2 

100 1000 

FREQUENCY IN CYCLES 


4 6 8 1 

lopoo 


Figure 28. Audibility of sonic noise from a fleet- 
type submarine at 3 knots and periscope depth, 
masked by shrimp noise. RD for presentation 
band shown: — 21 decibels. Character of prim- 
audible signal: hum or whirr. Primaudible fre- 
quency: 360 cycles. See Table 4, line 23. 


Figure 29. Audibility of sonic noise from a fleet- 
type submarine at 5 knots and periscope depth, 
masked by simulated deep-sea noise. RD for 
presentation band shown: — 14 decibels. Char- 
acter of primaudible signal : squeaking and 
rhythmic hiss. Primaudible frequency: 7 kilo- 
cycles. Arrow indicates fluctuating single-fre- 
quency component. See Table 4, line 7. 



Figure 30. Recorder traces of components in optimal 50-cycle band centered at 7 kilocycles for 
sonic noise from a 5-knot submarine and simulated water noise (Figure 29). Vertical distance be- 
tween lines represents power level change of 5 decibels. Total writing time was about 60 seconds. 
The relative level of signal and background corresponds to primaudibility. 



RESTRICTED 



100 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


nent tone at 7 kilocycles, there would be no 
substantial increase in the level of the signal 
at that frequency if the measurement had been 
made with a 400-cycle filter, which is the criti- 


but the signal spectrum was found to change 
markedly in level over the half-octave band be- 
tween 6.4 and 9.0 kilocycles; in other words, 
signal-to-noise ratios in the 50-cycle and the 



Figure 31. Time-frequency-intensity analysis for sonic noise from a 5-knot submarine (Figures 
29 and 32). 


cal band width at 7 kilocycles, but the level 
of the distributed noise spectrum would have 
been increased by 9 decibels (10 log 400/50 = 9 
decibels). Thus, the resultant signal-to-noise 



4681 2 4681 2 4681 

100 1000 10,000 
FREQUENCY IN CYCLES 


Figure 32. Audibility of sonic noise from a fleet- 
type submarine at 5 knots and periscope depth, 
masked by shrimp noise. RD for presentation 
band shown: — 15 decibels. Character of prim- 
audible signal: whine. Primaudible frequency: 
500 cycles. See Table 4, line 22. 


ratio, corrected to the appropriate critical 
band, is —1 decibel, rather than +8 decibels. 

Figure 38 (line 13 in Table 4) represents a 
borderline case. The optimal frequency band 
for this signal was centered near 8.6 kilocycles. 
Recognized as a “swish,” the primaudible com- 
ponent was definitely not a single frequency. 


half-octave bands did not agree. Furthermore, 
the signal-to-noise ratio of 1 decibel in the op- 
timal 50-cycle band is too low if the primau- 
dible component was narrower than 500 cycles, 
which is the critical band width at 8.6 kilo- 
cycles. To determine whether the critical-band 
criterion was met in this case, it would be 
necessary to use filters intermediate in width 
between 50 cycles and half an octave, and these 
were not available. Integration of power levels 
over the ten successive 50-cycle intervals in 
the signal which define the critical band cen- 
tered at 8.6 kilocycles is not a practicable sub- 
stitute for a single 500-cycle filter because the 
fraction of the energy passed by any of the 
50-cycle filters amounts to 10-^ of the overall 
signal energy, whereas the latter fiuctuates by 
at least 50 to 100 per cent during the course 
of a measurement. Thus, the experimental er- 
ror is of the order of the effect which it is 
desired to study. 

The results indicate that the recognition fre- 
quency at primaudibility is not a fixed charac- 
teristic of the signal but depends also upon the 
composition of the background. Thus, when 
the same signal is presented against two very 
different backgrounds, the optimal frequency 
band may shift from 0.2 to 8.6 kilocycles (com- 
pare Figures 38 and 40), or from 0.5 to 7.0 
kilocycles (compare Figures 29 and 32) . Simi- 
larly, the wide-band RD is not a fixed charac- 


LEVEL IN 50-CYCLE BAND IN DB 
ABOVE OVERALL LEVEL OF BACKGROUND 


UCDWR TESTS 


101 



Figure 33. Recorder traces of components in optimal 50-cycle band centered at 500 cycles for 
sonic noise from a 5-knot submarine and shrimp noise (Figure 32). Vertical distance between 
lines represents power level change of 5 decibels. Total writing time was about 20 seconds. The rela- 
tive level of signal and background corresponds to primaudibility. 



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Figure 34. Audibility of sonic noise from a sur- 
faced fleet-type submarine under way at 12 
knots, masked by simulated deep-sea noise. RD 
for presentation band shown: — 6 decibels. Char- 
acter of primaudible signal: tinny grinding and 
low rumble. Primaudible frequencies: 120 and 
1,400 cycles. See Table 4, line 9. 


Figure 35. Audibility of sonic noise from a sur- 
faced fleet-type submarine under way at 12 
knots, masked by shrimp noise. RD for presen- 
tation band shown: — 7 decibels. Character of 
primaudible signal: low rumble and whirr. Prim- 
audible frequencies: 120 and 1,400 cycles. See 
Table 4, line 24. 


kESTRIciiS n 



102 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


teristic of signals and may vary more or less 
widely and erratically from one background 
type to another (compare, for example, lines 1 
and 17 and also 2 and 18 in Table 4, to select 



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Figure 36. Audibility of sonic noise from an 
anchored S-class submarine, charging batteries 
with diesels, masked by simulated deep-sea noise. 
RD for presentation band shown: — 6 decibels. 
Character of primaudible signal: low-frequency 
rhythm. Primaudible frequency: 120 cycles. See 
Table 4, line 1. 


less intermodulation energy to the low-fre- 
quency region. It seems more likely that the 
help afforded by passing only the low-frequency 
components in such cases may be ascribed al- 



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Figure 37. Audibility of sonic noise from an 
anchored S-class submarine, charging batteries 
with diesels, masked by shrimp noise. RD for 
presentation band shown: — 10 decibels. Char- 
acter of primaudible signal : low-frequency 
rhythm. Primaudible frequency: 150 cycles. See 
Table 4, line 17. 


two cases at random). On the other hand, the 
signal-to-noise ratio in the optimal critical 
band is much more nearly a constant quantity. 

A few of the observations indicate that the 
recognition differential in the critical band may 
depend on background type to some extent. 
Thus, the same signals require 2 to 3 decibels 
more gain in the optimal critical band when 
the background is shrimp noise than when it 
is simulated ambient noise (compare Figure 
34 with Figure 35 and also Figure 36 with 
Figure 37). Similarly, it was observed that 
the recognition differentials (measured at the 
filter input) were 1 to 2 decibels more favor- 
able for various signals masked by shrimp 
noise when an 1,100-cycle low-pass filter was 
used, and that listening comfort was increased. 
While it has been suggested that some low- 
frequency masking results from intermodula- 
tion products (difference frequencies) of the 
high-frequency energy of shrimp crackle, it 
will be recalled that similar results were ob- 
tained with backgrounds containing much less 
high-frequency energy than shrimp noise (see 
Figure lOD), and therefore contributing much 



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Figure 38. Audibility of sonic noise from a 
7,000-ton freighter at 10 knots, masked by simu- 
lated deep-sea noise. RD for presentation band 
shown: — 12 decibels. Character of primaudible 
signal: rhythmic swish. Primaudible frequency: 

8.6 kilocycles. Shaded area indicates fluctuating 
components. See Table 4, line 13. 

most entirely to reduction of the loudness of 
the filtered sound, thereby permitting the op- 
timal component to be presented at a higher 
listening level. This is especially important in 
cases where the optimal frequency region lies 


^ stricteiT^ 


UCDWR TESTS 


103 


below 200 cycles, since acoustic leakage about 
the headphone cap, as well as smaller attenua- 
tion of room noise, tend to reduce listening ef- 
ficiency. On the other hand, use of low-pass 


ness of the critical-band concept, it should be 
observed that these sounds were, to some 
extent, arbitrary. Thus, the spectra shown in 
the figures represent electrical outputs from a 



Figure 39. Recorder traces of components in optimal 50-cycle band centered at 8.6 kilocycles for 
sonic noise from a 10-knot freighter and simulated water noise (Figure 38). Vertical distance be- 
tween lines represents power level change of 5 decibels. Total writing time was about 40 seconds. 
The relative level of signal and background corresponds to primaudibility. 


filters and increased listening level may fail 
to help when the optimal band lies at the lower 
frequencies, since headphones generally pro- 
duce serious distortion in this region, resulting 



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Figure 40. Audibility of sonic noise from a 
7,000-ton freighter at 10 knots, masked by 
shrimp noise; shaded area indicates fluctuating 
components. RD for presentation band shown: 

— 14 decibels. Character of primaudible signal: 
rhythmic chug. Primaudible frequency: 260 
cycles. See Table 4, line 26. 

in a confusing similarity between signal and 
background. 

While the sounds which were examined in 
the study under discussion indicate the useful- 


large variety of hydrophones and amplifiers. 
Hence the wide-band recognition differentials 
obtained with signal and background pairs from 
unmatched hydrophones may give an unreliable 
picture of the field situation in which, of course, 
signal and noise are transmitted by the same 
system. For example, lines 17 and 18 in Table 
4 show a wide band RD of —20 decibels for a 
submarine signal when the signal and back- 
ground were recorded through widely different 
hydrophones, and an RD of — 10 decibels when 
the same submarine signal was recorded 
through a hydrophone similar to that used in 
recording the noise. Clearly, the latter RD is 
the quantity of practical use. Unmatched 
recording hydrophones also account for the 
change in optimal component and improvement 
in RD shown by line 2 as compared with line 1 
in Table 4. 

Similarly, ground noise due to imperfections 
in the recording may introduce errors in rela- 
tive level at the higher frequencies.®’^® In addi- 
tion, no examples of sonic self-noise are in- 
cluded in the group of masking backgrounds 
studied in the present case. The possibility that 
strong tonal components, generated by the 
operation of auxiliaries, may frequently occur 
in self-noise poses a question which cannot be 
answered on the basis of the present findings. 


c 


RESTRICTED | 




104 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


since beats between tones in signal and noise 
probably render the simple critical-band con- 
cept inadequate (see, for example, Section 
2.1.2). Furthermore, the diminution in signal 
level with increasing range is invariably accom- 


tive samples ; in particular, the cases discussed 
down to this point give no adequate picture of 
the recognition of propeller sounds. The latter 
type of signal is discussed in the following 
section. 


BACKGROUND 




1 J I 1 1 IL 1 L ^ 1 1 Hill 1 ill ^ll 1 L ^l 1 ■ L ll rt ll 1 I- k * 


SIGNAL 

It T 

“ifirit — 1 — 1 — 1 — r 

1, Hill 



1 fii ,t 

ttwwt'-wl tf l| 

iptliaifF 


— rir~ 







y— 



Figure 41. Recorder traces of components in optimal 50-cycle band centered at 260 cycles for 
sonic noise from a 10-knot freighter and shrimp noise (Figure 40). Vertical distance between 
lines represents power level change of 5 decibels. Total writing time was about 30 seconds. The rela- 
tive level of signal and background corresponds to primaudibility. 


panied, in practice, by selective attenuation, 
which weakens the higher frequencies to a 
greater extent than the low frequencies. The 
frequency composition of recorded signals used 
in tests of this kind, however, does not vary in 
a comparable way as a function of signal level. 


^ Sonic Propeller Sounds 

The information at present available con- 
cerning recognition of sonic propeller sounds is 
somewhat limited. Some quasi-held studies 
have been made with these sounds and are 



Figure 42. Time-frequency-intensity analysis for sonic noise from a 10-knot freighter (Figures 
38 and 40). 


It seems quite likely, therefore, that in opera- 
tional listening the prominent high frequencies 
in shrimp noise would have but little inhuence 
in masking sonic ship signals, since relatively 
little high-frequency energy is received from 
the latter at practical ranges. Finally, it is im- 
possible to say that the spectra of the various 
sources employed in these tests are representa- 


described in reference 13. In the present sec- 
tion, some preliminary UCDWR tests, made 
partially with simulated signals, are described. 
The propeller simulations were amplitude- 
modulated thermal noise which had a spectrum 
slope of 0 decibel per octave. One of the mask- 
ing backgrounds used was simulated deep-sea 
ambient, that is, unmodulated thermal noise 


UCDWR TESTS 


105 


with a slope of —6 decibels per octave. Both 
sounds are fairly good facsimiles of the actual 
ones and probably offer a useful guide to the 
factors affecting audibility of propeller cavita- 



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Figure 43. Audibility of sonic noise from a 
2,870-ton destroyer, masked by simulated deep- 
sea noise. RD for presentation band shown : 

— 11 decibels. Character of primaudible signal: 
low-pitched tone with propeller rhythm. Prim- 
audible frequency: 250 cycles. See Table 4, 
line 14. 

tion. Tests were conducted in which both signal 
and noise extended over the entire 0.1- to 10- 
kilocycle band, and also in which filters were 
introduced to restrict the frequency range of 


Figures 62 and 63. It will be observed from 
these figures that the sole difference between 
background and signal was the modulation of 
the latter. Thus, the listening problem which is 



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Figure 44. Audibility of sonic noise from 
tanker, masked by simulated deep-sea noise; the 
arrow indicates a fluctuating single-frequency 
component. RD for presentation band shown: 

— 9 decibels. Character of primaudible signal: 
low rhythmic noise and whine. Primaudible fre- 
quency: 680 cycles. See Table 4, line 12. 

presented is that of distinguishing between the 
fluctuations normally present in the back- 
ground and those caused by the presence of the 
signal. Since the time pattern of the back- 



Figure 45. Recorder traces of components in optimal 50-cycle band centered at 680 cycles for sonic 
noise from a tanker and simulated water noise (Figure 44). Vertical distance between lines repre- 
sents power level change of 5 decibels. Total writing time was about 30 seconds. The relative level 
of signal and background corresponds to primaudibility. 


the signal alone or of the entire mixture. The 
other masking background used was unmodu- 
lated wide-band thermal noise with a slope of 
0 decibel per octave. Time patterns of these 
artificial signal and noise types are shown in 


ground is random, any rhythm in the signal 
provides a useful cue. 

The general psychoacoustic data discussed in 
Chapter 2 provide a basis for understanding 
and evaluating these tests on the masking of 


RESTRICTED 


1 > 


106 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


propeller sounds. Previous observations (Sec- 
tion 2.2.2) indicate that rhythmic fluctuations 
can be readily perceived if the modulation rate 
does not exceed 10 cycles; at higher rates, the 


sity increment of about 1 decibel can be just 
recognized, at the optimal modulation rate of 3 
cycles. Under ideal conditions very much 
smaller intensity increments can be perceived 



Figure 46. Time-frequency-intensity analysis for sonic noise from a tanker (Figure 44). 


modulation is heard as a ‘‘flutter” and becomes 
more difficult to discern. For very low modula- 
tion rates, less than about 1 per second, a 
memory image of the intensity is required; 



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Figure 47. Audibility of sonic noise from a 45- 
knot torpedo (3-turbine type), masked by simu- 
lated deep-sea noise; the arrow indicates a fluc- 
tuating single-frequency component. RD for 
presentation band shown: — 11 decibels. Char- 
acter of primaudible signal: high-pitched whine 
or hum. Primaudible frequency: 970 cycles. See 
Table 4, line 15. 


— even as small as 0.1 decibel. As already 
pointed out in Section 2.2.2, recognition of 
these very small increments is not likely in 
most practical situations. The data obtained in 
the masking studies described here give sup- 
port to this view. 

A minimal intensity increment of 1 decibel 
corresponds to a recognizable signal which is 
considerably weaker than the background. The 
recognition differential for such a signal may 
be estimated as follows. Let I be the average 
overall intensity of the background, and let ip 
and i,n be the peak and minimum intensities of 
the signal. If m is the percentage amplitude 
modulation of the signal, then, since intensity 
is proportional to the square of the amplitude. 



Suppose recognition occurs when the maxi- 
mum intensity I-\-ip of the signal -background 
mixture is 1 decibel above the minimum inten- 
sity, I 4- itn- Since 10 log 1.25 equals 1 decibel, 
the condition for detecting the change is 


since this image fades with time, recognition 
tends to deteriorate. It was also pointed out 
that, when both signal and noise are wide-band 
sounds with identical spectra, the recognition 
of the signal depends only on the general in- 
crease of intensity which it produces over the 
entire band. Under these conditions an inten- 


1.25 


1 + ip , 
1 + Tm 


( 10 ) 


If in is eliminated from these two expressions, 
we obtain after some manipulation 


20m 


(1 + m )2 


( 11 ) 




ESTRTCTED 


UCDWR TESTS 


107 


Also, the mean intensity i is simply /2, 

which, together with equation (9), gives 


. 1 + m2 

* “ (1 + to )2 ■ 


fl2) 


Thus, for 100 per cent modulation (m = l), 7 is 
four times giving a peak intensity 6 decibels 
below the average noise background. Since the 
mean signal intensity is 3 decibels below the 
peak for 100 per cent modulation (see equa- 


level is 4.1 decibels below the noise, giving a 
mean level RD of —6.0 decibels. These rela- 
tionships are portrayed in Figure 54, where 
the signal is shown in relation to the noise 
background. At the top of each diagram the 
combination of noise and modulated signal is 
drawn, with a total variation of 1 db as as- 
sumed. 

Three limitations are apparent in the above 
analysis. In the first place, the signal and back- 


o 

•iC 


o 

lU 

cc 


3 

2 

I 

0 







0 .2 .4 .6 


.8 1.0 U2 1.4 1.6 1.8 2.0 2.2 2.4 


TIME IN SECONDS 


Figure 48. Time-frequency-intensity analysis for the signal and the' signal-background mixture 
shown in Figure 47. For the signal-background mixture (lower trace), relative intensities have 
been adjusted to primaudibility. 



3 — 


2 — 



kO 1.2 1.4 U6 1.8 2.0 2.2 2,4 

TIME IN SECONDS 


tion (12)), the RD based on average signal 
level is, in this optimal case, —9 decibels. The 
latter conclusion is not obvious from a power 
level record of the signal since the minimum 
intensity of a sound with 100 per cent ampli- 
tude modulation is zero, or an infinite number 
of decibels below reference, as indicated by the 
dotted portion of the curve for m = l which is 
given in Figure 54A. For 50 per cent modula- 
tion, the peak is 9/31 of the average noise, or 
5.4 decibels below; the corresponding RD based 
on average signal intensity is —7.9 decibels. 
For 30 per cent modulation the peak signal 


ground must be wide-band noises, with random 
phases. If both signal and background were 
pure tones, for example, the intensity of the 
two would be obtained by adding their ampli- 
tudes and squaring rather than adding intensi- 
ties; the RD in this hypothetical case may be 
shown to be —24 decibels (see Section 2.1.2). 
Since, in fact, the signal and background are 
very unlikely to have common phases at any 
frequency, the results shown in Figure 54 
should be moderately realistic. In the second 
place, the above result does not hold for weak 
modulations — less than about 10 or 15 per cent. 


108 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


CD 

O 


O 

z 

< 

OD 


UJ 

_l 

o 

o 

g 

z 


3 

o 

cr 

s 

o 

<x 


UJ 

> 

o 

m 

< 




BACKGROUND 



















s 








A 

X. 




















V 






. 

.A 







SIGNAL 

y 

V/ 










l\ 









\ 









\ 





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Figure 49. Audibility of sonic noise from a 30- 
knot torpedo (electric type), masked by simu- 
lated deep-sea noise. RD for presentation band 
shown: — 17 decibels. Character of primaud- 
ible signal: hum and whine. Primaudible fre- 
quency: 660 cycles. See Table 4, line 16. 


FREQUENCY IN CYCLES 

Figure 50. Audibility of sonic noise from a Ger- 
man Type VIIC submarine (GRAF) at 2 V 2 
knots and periscope depth, masked by simulated 
deep-sea noise; the shaded area indicates fluc- 
tuating components. RD for presentation band 
shown: — 11 decibels. Character of primaudible 
signal : low rhythmic bubbling. Primaudible fre- 
quency: 300 cycles. See Table 4, line 10. 



0 *2 .4 ,6 ,8 bO 1.2 1.4 1.6 1.8 2.0 2.2 2.4 


TIME IN SECONDS 


Figure 51. Time-frequency-intensity analysis for sonic noise from a 2.5-knot submarine (Fig- 
ure 50) . 



Figure 52. Time-frequency-intensity analysis for the signal-background mixture shown in Figure 
50, with relative intensities adjusted to primaudibility. 


UCDWR TESTS 


109 


Such a small variation would be difficult to 
detect, and the signal would be detected by 
sweeping on and off target rather than by lis- 
tening for characteristic modulation at each 
bearing. Finally, both signal and background 
must have parallel spectra over a wide range of 



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Figure 53. Audibility of sonic noise from a con- 
voy (13 merchant vessels, 6 large tank lighters, 
and 3 PC escorts), masked by simulated deep-sea 
noise; the arrow indicates a fluctuating single- 
frequency component. RD for presentation band 
shown: — 12 decibels. Character of primaudible 
signal: hum or whine. Primaudible frequency: 

390 cycles. See Table 4, line 11. 

frequencies with no tonal components in the 
signal, so that recognition occurs in a wide 
band. 

Propeller sounds are usually composed of 
wide-band cavitation noises with a spectrum 
similar to the background over a fairly wide 
band, usually several kilocycles or more. If 
attention is confined to this band, the results 
derived may be expected to be applicable pro- 
vided that the modulation is 30 per cent or 
more. Thus the spectrum differential, or the 
spacing between the signal and noise spectra 
at primaudibility, for such modulated sounds 
should be between —5 and —9 decibels, pro- 
vided that the modulation rate lies between 1 
and 10 cycles (see Figure 64) and provided 
that tonal components (or other peaks) in dif- 
ferent parts of the signal spectrum are not the 
primaudible components. Also, the background 
noise level must be reasonably steady for this 
result to hold. If the time pattern of the back- 
ground noise is similar to that of the signal, as 
may perhaps occur with certain types of self- 


noise, a spectrum differential more nearly equal 
to zero would be expected. More usually, how- 
ever, the background is not modulated and a 
spectrum differential between —5 and —9 deci- 
bels may be expected. 

The available observations support this 
analysis in a number of situations. It has al- 
ready been noted in Section 4.1.3 that the Brit- 
ish data are in agreement with these results. 
When modulated propeller sounds could just be 
heard in the presence of wide-band masking 
noise, the smallest distance between the two 
spectra was 6 to 10 decibels. While signal and 
background spectra did not have identical 
shapes, they were sufficiently parallel above 
2 kilocycles for the foregoing analysis to be 
relevant, and thus the agreement can be taken 
as confirmation of the theory. As already indi- 
cated in the discussion of Figure 54, the arith- 
metic mean of two intensities a and h is not 
generally equal to the intensity c which is ob- 
tained by averaging the intensity levels 10 log 
a and 10 log h. The latter process gives (10 log 
0 . + 10 log h)/2 = 10 log \/ah. In other words, 
the derived quantity corresponds to the geo- 
metric rather than the arithmetic mean. Con- 
sequently, use of the mean signal level, read 
from a power level trace, may yield somewhat 
lower values for the spectrum differentials than 
estimated above. 

The UCDWR sonic data also show agreement 
with expectation. Thus, it was found that a re- 
current change in the intensity of a wide-band 
sound can be detected when the ratio of maxi- 
mum to minimum intensity equals 1.25, or 1 
decibel, and that this change can be detected 
equally well when it is produced either by 
modulating a single thermal noise source or by 
adding a modulated thermal noise to an un- 
modulated one having identical band width and 
spectrum slope. 

Similar results were obtained with ship 
sound recordings. Figures 55 and 56, for ex- 
ample, illustrate the relation at primaudibility 
between the spectra of a submerged submarine 
and of simulated deep-sea ambient. In this 
case, the primaudible band included the fre- 
quency region between 2 and 7 kilocycles. 
Within this region, the average signal level was 
12 decibels below average noise, and the maxi- 


RESTRICTED 


no 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


mum signal level 6 decibels below noise. The 
primaudible signal perceptibly raised the total 
loudness and was heard as a rhythmic thrash 
with a clearly audible modulation of about 2 
beats per second. 

It was found, when the components of the 


Figure 57 sheds some light on the preceding 
observations. The three rows of recorder traces 
show time patterns of simulated propeller 
noise, flat thermal noise background, and a 
mixture of both. The latter was presented to 
the recorder at the same signal-to-noise ratio 



AL = 10 log [(/ + ip) / (/ + im)] = recurrent variation in level of mixture (1 db) 

Al = 10 log (ip / im) = recurrent variation in level of signal 

I = Mean intensity of unmodulated background (reference intensity) 
i = Mean intensity of modulated signal 
m = Percent amplitude modulation of signal 
ip = Recurrent peak intensity of modulated signal = i (1 + m)2, 
since intensity is proportional to square of amplitude 
im = Recurrent minimum intensity of modulated signal = i(l — m)2 

Figure 54. Relative intensity levels of a sinusoidally modulated signal and of the signal-back- 
ground mixture. Lower curves represent time patterns of signals; upper curves represent time 
patterns of mixtures. 


signal-background mixture with frequencies 
between 2 and 7 kilocycles were transmitted 
through progressively narrower band-pass fil- 
ters, that the primaudible signal level rose to 
continuously higher values. With a 50-cycle 
band-pass filter centered near 5 kilocycles, the 
signal was primaudible when its average level 
was equal to the average noise level, in other 
words, when the maximum signal level ex- 
ceeded the noise. When listening through the 
50-cycle filter, the amount of random fluctua- 
tion perceived by the ear was noticeably 
greater than when listening in the wider bands. 
In addition, the mixture had an unpleasant 
tonal quality when passed by the narrow filter, 
the amplitude modulation was difficult to dis- 
cern, and no reliable turn count could be ob- 
tained at primaudibility. 


for all the admittance band widths illustrated. 
The particular signal-to-noise ratio used is sev- 
eral decibels higher than is needed for prim- 
audibility when the mixture is heard through 
the 50-cycle filter. The input levels of signal 
and noise were held constant, and, since filter- 
ing reduces the energy fed to the recorder, the 
gains were adjusted to keep the traces in the 
same relative positions on the paper. Such 
gain changes do not affect the relative amount 
of fluctuation shown for a particular sound. 

The observed fluctuation does increase with 
decreasing band width. This observation may 
account, at least in part, for the impaired audi- 
bility of the submarine cavitation which was 
just discussed, and it indicates that restric- 
tions of system band width must be made with 
caution. This same problem is touched upon in 


V RESTRICTED 


UCDWR TESTS 


111 


various other connections ; see, for example, the 
discussion in Chapter 8. It may be mentioned 
here, however, that a sharply tuned system has 
a longer response time than one which is un- 



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Figure 55. Audibility of sonic noise from a fleet- 
type submarine at 120 rpm (6 knots) and peri- 
scope depth, masked by simulated deep-sea noise. 
Character of primaudible signal : rhythmic 
thrash, twice each second. 


tuned or broadly tuned, and the noise fluctua- 
tions in such a tuned system are less rapid than 
in a system passing a broad band of frequen- 
cies. In other words, admittance band width 
determines the ‘'resolution time’" of the system. 

The recorder traces under discussion show 
the influence of two distinct resolution times: 
that of the electrical system and that of the 
recorder. The latter instrument indicates the 
average d-c voltage applied to its terminals 
during its own response time. The greater 
“smoothness” of the trace obtained with re- 
duced electrical selectivity means that the 
broader systems feed a greater number of fluc- 
tuations to the recorder during its resolution 
time and that the larger sample shows less 
mean deviation from the average noise level. 
To a first approximation, the degree of fluctua- 
tion should be inversely proportional to the 
square root of the band width. 

One further point is worth noting. It is evi- 
dent that if the critical bands were completely 
independent, narrow Altering should not have 
impaired recognition of the submarine cavita- 
tion. The signals received in each “critical-band 
filter” of the ear are therefore not heard sepa- 



Figure 56. Recorder traces of components in optimal band for the sonic noise from a 6-knot sub- 
marine and the simulated deep-sea noise shown in Figure 55. The relative level of signal and back- 
ground corresponds to primaudibility. 


112 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


rately, but as a unit. The ear’s response to the 
mixture admitted by the 9-kilocycle band seems 
to involve some fusion of the impulses received 
by various parts of the basilar membrane and 


tance of about 120 yards (see Figure 9 in 
Chapter 3). The masking background was 
simulated deep-sea noise, and recognition oc- 
curred when signal and background levels were 





A. ARTIFICIAL SIGNAL : THERMAL NOISE MODULATED 6 DB AT 3 CYCLE RATE 


H 



} 5 00 1 SEC 











hr%MM 






8. MASKING NOISE *. THERMAL NOISE — UNMODULATED 



1 1 











5 00 1 SEC 












111, 1 illi LiiH 

it 

bill 1 1 1 il.A t 

ii yii> If*. 11 A 

TF 




1 

pSfflV 

\nm 

W 





If— 










C. NOISE 

Figure 57 . 


AND SIGNAL MIXED AT ABOVE LEVELS 

Power level recorder traces showing time-amplitude patterns of thermal noise. 


is apparently more like the recorder trace for a 
9-kilocycle band than it is like the trace for a 
band 50 or 235 cycles wide, although the latter 
bands are approximately the width of the aural 
critical bands. This integrating property of the 
ear is also apparent in the observed masking of 
FM pulses (see Section 8.1.3). 

The foregoing discussion applies to masking 
of propeller noise when this consists of modu- 
lated cavitation sounds. Propeller noise is ap- 
parently not always of this character, however, 
as evidenced by the following two examples. 
Figure 58 shows the signal-background rela- 
tionship at primaudibility for an aircraft car- 
rier moving at 15 knots and recorded at a dis- 


approximately equal at all frequencies in the 
entire listening band. This case stands at the 
opposite extreme from Figure 55. Such varia- 
bility in the recognition of propeller sounds is 
perhaps the result of intrinsic variability asso- 
ciated with the presence or absence of strong 
cavitation. Thus the submarine whose spectrum 
is shown in Figure 55 was proceeding at 6 
knots, which is well above the cavitation 
threshold at periscope depth, while the carrier 
was moving at a speed which produces no 
marked cavitation in her class of vessels.^^ It 
should be observed, however, that the listening 
test described by Figure 58 was performed 
without benefit of the sweep pattern which is 


lRESTPICTEDM 



UCDWR TESTS 


113 


introduced by training a directional hydrophone 
across the target bearing. With optimal hydro- 
phone sweep rates (see Section 5.1), it should 
be possible to detect a source like the carrier 



I 2 4681 2 4681 

'400 1000 10,000 


FREQUENCY IN CYCLES 

Figure 58. Audibility of sonic noise from 15- 
knot aircraft carrier A, masked by simulated 
deep-sea noise. RD for presentation band shown ; 

— 3 decibels. Character of primaudible signal : 
propeller sounds. 

when the intensity increment produced in the 
transit is about 1 decibel, instead of the 3 deci- 
bels indicated by Figure 58, and, hence, when 
the signal is about 6 decibels below average 
noise. For signals like that generated by the 
submarine, sweeping the target bearing should 
produce little additional improvement; indeed, 
it is likely that hydrophone sweep rates should 
be diminished for well-modulated signals in 
order to obtain optimal results. 

The spectrum of another carrier of the same 
class, operating under similar conditions, is 
shown in Figure 3 of Chapter 3. The dominant 
tone at 1,100 cycles was produced by a ‘"sing- 
ing” propeller, and was amplitude modulated 
at the screw rate. The “cavitation peak” below 
300 cycles also appeared in the spectrum of the 
output of the first carrier when measured at 
bow and quarter aspects (see Figure 9 of Chap- 
ter 3) ; this trend has been ascribed to source 
directivity. 

In the presence of simulated ambient noise, 
the second carrier was detected as a faint whine. 
The relation between signal and noise spectra 
at primaudibility, as measured in 50-cycle 
bands, is shown in Figure 59. The critical-band 


criterion predicts that a tone at 1.1 kilocycles 
will be primaudible when its level in a 50-cycle 
band is just equal to the distributed noise in 
this band. The apparent discrepancy is due to 
the fact that the spectrum, as plotted, gives the 
average, rather than the maximum level of 
the tone. It is evident that the second carrier 
is much more vulnerable to detection than the 
first, and may be better camouflaged acousti- 
cally through a simple modification of propeller 
design. 

The carrier spectrum shown in Figure 59 has 
been recomputed to show how it would appear 



FREQUENCY IN CYCLES 


Figure 59. Audibility of sonic noise from 15- 
knot aircraft carrier B, masked by simulated 
deep-sea noise. RD for presentation band shown : 

— 12 decibels. Character of primaudible signal ; 
whine. 

if the analysis had been made with octave fil- 
ters exclusively. With such an analysis, the 
relation between signal and background spectra 
at primaudibility would be given by Figure 60. 
Comparison of the latter with Figures 6 
through 23 indicates the general similarity of 
the British and the San Diego results, when 
allowance is made for differences in technique 
of analysis. It also demonstrates, incidentally, 
that wide-band analysis is a rather blunt tool 
for some purposes. 

The observations discussed so far in this sec- 
tion apply exclusively to the situation in which 
signal and noise spectra have closely similar 
slopes over a wide frequency band. Since at- 


^ CRK'I’RICTE 


114 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


tenuation modifies the 6 decibels per octave 
slope of the propeller cavitation spectrum and 
since the self-noise received in ship-mounted 
hydrophones usually has a slope of between 9 



> 2 4681 2 4681 

100 1000 10,000 


FREQUENCY IN CYCLES 

Figure 60. Spectra for the sonic noise from air- 
craft carrier B and simulated deep-sea noise 
shown in Figure 59, when the energy is mea- 
sured in octave bands. 

« 

and 12 decibels per octave, it will not in gen- 
eral be true that signal and noise spectra are 
parallel over a wide frequency band. It is 
useful, therefore, to examine some further ob- 
servations made with wide-band sounds gen- 
erated in the laboratory. 

The masking background in these tests was 
simulated deep-sea ambient, which had a slope 
of —6 decibels per octave and extended from 
0.1 to 10 kilocycles (see Figure 62). The sig- 
nals were 100 per cent amplitude modulated at 
a rate of 3 cycles (see Figure 63) and were 
obtained from a wide-band source of thermal 
noise with a flat spectrum (amplitudes equal at 
all frequencies) . Each test was made with a 
different band-pass filter in the signal channel. 
These filters varied from half an octave to sev- 
eral octaves in width, and their midfrequencies 
fell at representative points in the interval be- 
tween 500 and 7,000 cycles. 

The signal-background relationship at prim- 
audibility which was typically observed in 
these tests is shown in Figure 61. The prim- 


audible group of signal components was invari- 
ably centered at the high-frequency limit of the 
signal pass band; and the maximum signal-to- 
average noise ratio in this optimal band was 
very nearly —2 decibels in all the cases studied. 
This signal-to-noise ratio is essentially the 
same as that found for fluctuating tonal com- 
ponents. In other words, the critical-band 
criterion applies also to the situation in which 
the primaudible sound consists of a narrow 
band of frequencies, and the ability to discrimi- 
nate small differences of loudness is less than 
for the case in which the primaudible compo- 
nent is heard in a wide band (see Figure 55). 
The optimal signal-to-noise ratios observed in 
other masking tests in the series illustrated in 
Figure 61 showed a slight dependence on the 



FREQUENCY IN CYCLES 


Figure 61. Audibility of modulated thermal 
noise, passed by an octave filter, masked by 
simulated deep-sea noise. RD for presentation 
band shown: — 18 decibels. 


frequency of the primaudible component and 
possibly also on sensation level. Since this set 
of tests was of a preliminary nature, the results 
are not precise enough to indicate whether the 
slight differences in optimal signal-to-noise 
ratio are more closely related to the critical 
band width function or to the loudness incre- 
ment function. In any case, the observations 
discussed in the present section indicate that 
the width of the primaudible band affects the 
audibility of signals. 

The rate of modulation may also be expected 
to affect the RD for a wide-band or narrow- 
band sound. Mo systematic studies of the role 
played by modulations found in underwater 
sounds have as yet been made. In lieu of this, 
however, a rough estimate of what may be ex- 


^ Restricted 


1 


UCDWR TESTS 


115 


pected in practice for wide-band sounds may be 
obtained from the data in Figure 16 of Chap- 
ter 2. This estimate, computed for a signal and 
a noise background with similar spectra, heard 


level of 25 decibels were used, since the total 
loudness of wide-band signals tends to limit 
the possible gain available in the optimal band. 
Furthermore, the data presented in Chapter 2 



Figure 62. Time-frequency-intensity analysis for simulated water noise. 


in a wide frequency band, is given in Figure 64. 

The method of calculation has already been 
explained in connection with Figure 54. The 
ordinate, labeled “spectrum differential,” shows 
the expected spacing, when the signal is just 
detectable, between the essentially parallel por- 


imply that the loudness increments selected for 
the present estimate are large enough to have a 
detection probability of 100 per cent rather 
than of 50 per cent, and even smaller changes 
can be detected when the sound is heard with 
both ears, as in most practical listening. As 



0 ,2 






LO 1.2 1,4 

TIME IN SECONDS 


I 

1.8 2,0 2.2 2.4 


Figure 63. Time-frequency-intensity analysis for 100 per cent amplitude-modulated thermal noise. 


tions of the signal and noise spectra which lie 
in the primaudible frequency band. The 
abscissa gives propeller rpm, and the curve 
applies to 50 per cent amplitude modulation. 

This curve has been drawn for the situation 
in which both signal and noise spectra are ex- 
pressed in terms of average values of rms level 
because that has been the usual practice in mea- 
suring underwater sounds up to the present. 
Several assumptions have been made in prepar- 
ing Figure 64. To begin with, the data in Fig- 
ure 16 of Chapter 2 pertaining to a sensation 


already pointed out, the more conservative 
values have been chosen, since the intensity 
limen (smallest perceptible intensity incre- 
ment) varies with the primaudible frequency 
and because various other unfavorable factors 
may enter the field situation. In addition, the 
values of the intensity limen employed here 
were derived from studies with tones. General 
considerations, however, as well as a limited 
amount of experimental evidence (see Section 
2.2.2) indicate that limens for tones and for 
distributed sounds are comparable in magni- 


RESTRICTE 




116 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


tude, although the facts have not been estab- 
lished in detail. Finally, a modulation value of 
50 per cent has been adopted as typical of well- 
modulated cavitation ; for comparison, it should 



PROPELLER BEATS PER MINUTE 


Figure 64. Estimated effect of the rate of 
amplitude modulation on the detectability of pro- 
peller sounds. This estimate assumes 50 per cent 
amplitude modulation and a wide band of prim- 
audible frequencies. 


be noted that the curve in Figure 64 would be 
shifted up — to smaller negative values — by 
about 2 decibels for the case of 30 per cent 
modulation and would be shifted down by about 
1 decibel for 100 per cent modulation. 

Several practical points should be noted in 
connection with Figure 64. It applies primarily 
to recognition of propeller cavitation sounds 
and has no meaning below the cavitation 
threshold. The latter may occur at widely dif- 
ferent propeller rates, depending on circum- 
stances. In fact, it seems probable that the 
degree as well as the rate of modulation is a 
function of propeller rpm. While the indicated 
range of differentials checks fairly well with 
the available observations, it seems likely that 
the curve relating spectrum differential to screw 
rate for a particular vessel will, in general, 
follow a somewhat different course from that 
shown in Figure 64ri)ecause of the influence of 
at least four other factors. 

In the first place, there is evidence^^ that the 
slope of the cavitation spectrum is a function of 
ship speed. Secondly, some propeller sounds 
seem to be characterized to a greater or lesser 
degree by the presence of frequency modula- 
tion ; this is heard as a periodic swish,^ and, in 

g While no systematic observations are available, 
preliminary results obtained at UCDWR indicate that 
the ear does not readily distinguish between frequency 
and amplitude modulations in propeller sounds. 


an analysis, shows up as a recurrent change in 
amplitude of the components admitted by each 
of a group of narrow filters tuned to neighbor- 
ing frequencies (see Figure 12 in Chapter 3). 
The effect upon audibility of such a shifting 
spectral prominence depends on factors which 
have not yet been investigated. 

The third factor referred to above is back- 
ground type; the breaking of surf and similar 
sounds showing fairly long and regular recur- 
rence times may counteract the effects of am- 
plitude modulation in the signal. In that event, 
sweeping a directional hydrophone through the 



-30 -28 -26 -24 -22 -20 -18 -16 -14 -12 -10 -8 -6 


RATIO OF average SIGNAL TO AVERAGE NOISE IN DB 


LEVEL 

IN 

PHONS 

80 PER CENT 
RECOGNITION 
PROBABILITY 

50 PER CENT 
RECOGNITION 
PROBABILITY 

DIFFERENCE 
IN DB 

n 

80 

-16.3 

-18.5 

2.2 

2.7 

40 

-12.8 

-15.2 

2.4 

2.5 

2 1 

-11.5 

-13.0 

1.5 

4.0 

1 1 

-1 1 . 1 

-9.9 

1.2 

5.0 


Figure 65. Estimated effect of loudness level on 
the probability of detecting propeller sounds 
with 50 per cent amplitude modulation in the 
presence of a steady background. 

target bearing may help, although this device 
has a number of limitations (see Chapter 5). 
In general, hydrophone sweep should help most 
when the propeller modulation rate is either 
well below or well above the optimal rate of 180 
rpm (see Figure 64). 

The fourth factor is the loudness level at 
which the signal and background are heard. 
The estimated effect of loudness level (when 


UCDWR TESTS 


117 


signal and background have similar frequency 
compositions) is shown in Figure 65, which has 
been recomputed from reference 12 by assum- 
ing that the intensity increments indicated in 
the latter were produced by the introduction of 
a signal, that is, by the method already de- 
scribed in the discussion of Figure 54. The 
very low recognition differentials shown in the 
figure correspond to the very small intensity 
increments which can be perceived under ideal 
conditions. Under practical conditions the 50 
per cent RD will presumably correspond more 
nearly to a 1-decibel intensity increment. It 
may be noted that, when cavitation sounds are 
heard in only part of the spectrum, it is the 
contribution of these spectral components to 
the total loudness that is significant. Intense 
components at low frequencies, for example, 
may add to the total loudness without modify- 
ing the effective loudness of the cavitation 
sounds. 

Two effects are shown in Figure 65: (1) by 
increasing the listening level from 11 to 80 
decibels, the computed 50 per cent recognition 
differentials are improved by 9 decibels or 
more, in other words, smaller changes of in- 
tensity can be detected at the higher levels; 
(2) the computed transition curves are not only 
shifted but their slopes (see Section 4.1.4) also 
change more or less progressively, so that dis- 
crimination deteriorates more rapidly (with 
diminishing signal-to-noise ratio) at the low 
gain settings than at the high. The magnitudes 
of these changes are approximately independent 
of the assumed degree of signal modulation; 
thus, each curve is shifted to the left by about 
1 decibel for the case of 100 per cent modula- 
tion and to the right by about 2 decibels for the 
case of 30 per , cent modulation. The derived 
spectrum differential given in Figure 65 for 
approximately 100 per cent detection at a loud- 
ness level of 21 decibels would be expected to, 
and does, agree with the differential shown in 
Figure 64 for a rate of 180 rpm and a sensa- 
tion level of 25 decibels. An estimated sensa- 
tion level of 20 decibels within the optimal band 
typifies the six tests on audibility of propeller 
cavitation illustrated in Figures 16 through 18. 
The spectrum differentials observed in those 
tests are in good accord with expectation (see 


column 4 in Table 3). The components outside 
the optimal bands in these cases raised the 
total loudness to 70 phons; higher gain settings 
in tactical listening are uncomfortable, ineffi- 
cient, and possibly injurious. Since restriction 
of the presentation band is unwise during 
search, current listening techniques provide no 
simple means for securing the potential im- 
provement indicated in Figure 65. While track- 
ing a target, bearing accuracy can probably 
be improved by dropping the gain or by using 
high-pass filters; Figure 65 implies that the 
latter alternative is preferable, since auditory 
discriminations are finer at the higher sensa- 
tion levels. On the other hand, the transition 
curves in Figures 16, 17, and 18 show varia- 
tions of slope despite the approximate con- 
stancy in sensation levels of the optimal com- 
ponents, and the transition curves in Figures 
6 through 15 show little dependence on listen- 
ing level. The factors affecting the slopes of 
transition curves are of practical interest, but 
no definite conclusions would appear warranted 
without further experimental investigation. 

Supersonic Propeller Sounds 

The test materials used in supersonic recog- 
nition studies were recordings of ship and sub- 
marine sounds, self-noise, and ambient noise 
with and without shrimp crackle. Such sound 
sources are commonly encountered in super- 
sonic listening. The supersonic receivers used 
to obtain the reproductions were practical field 
installations. They admitted the frequencies 
between 23 and 25 kilocycles, which were het- 
erodyned before recording, so that the recorded 
band, that is, the listening band, was essen- 
tially flat between 0.1 and 2 kilocycles (see 
Figures 66 through 75). These supersonic and 
audio-frequency bands are typical in super- 
sonic listening. 

Most supersonic sounds have continuous 
spectra, although tonal components are some- 
times observed. Furthermore, the spectra of 
the sounds-in-the-water, where these are dis- 
tributed sounds, are essentially flat over the 
interval from 23 to 25 kilocycles. Thus, deep- 
sea ambient and propeller cavitation have 
slopes of about —6 decibels per octave in this 



118 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


frequency region, but the band between 23 and 
25 kilocycles extends over such a small fraction 
of an octave that the total change in level is 
negligible. The essential flatness of such spec- 
tra is not appreciably modifled by selective at- 
tenuation in transmission over relatively short 
ranges. Similarly, the discrimination of most 
hydrophones against background noise changes 
very little between 23 and 25 kilocycles (see 
Section 3.3). It follows that the changes in 
level observed in the spectra given in Figures 
66 through 75 are due principally to variations 
of system response with frequency. 

These variations arose from two main 
sources. In the first place, it was necessary to 
use several different types of gear. Secondly, 
the various installations were provided with 
i-f and audio-frequency filters ; their band 
widths, as well as inclusive system response 
over the nominal band widths, could not be very 
precisely duplicated from installation to instal- 
lation and from time to time. Obviously, the 
problem of fidelity is not peculiar to the re- 
cording of supersonic sounds. It is an impor- 
tant problem in all recognition studies, since 
signal and background recorded at different 
times and with different installations may give 
misleading results if mixed together in a lis- 
tening test. In connection with these test ma- 
terials, it should also be noted that^ a “limiting 
amplifier in the film recording process may 
have reduced the amplitude of the modulations 
which were recorded.’' The opinion is ex- 
pressed,^ however, that this amplitude distor- 
tion was probably not large. On the other 
hand, the quality of the recordings is rated no 
better than “fair” in most cases (see Table 5). 

The apparatus used in these tests is shown 
diagrammatically in Figure 1. The backgrounds 
were heard at a level of about 60 phons, and 
the random method of signal presentation was 
followed in obtaining the results shown in 
Table 5, and in Figures 66 through 75. Table 
5 is subdivided according to the type of mask- 
ing background ; effects of changing the presen- 
tation method are described in Section 4.2.6. 
To offset the influence of differences in the fre- 
quency response of the receivers employed to 
obtain the recordings, a 200-cycle high-pass and 
a 1,600-cycle low-pass filter were ordinarily in- 


serted in the signal channel. This is indicated 
by the arrows in Figures 66 through 75, while 
the dashed lines show the compositions of the 
unfiltered signals. Use of the filters was de- 



4681 2 4681 2 4681 

100 1000 10,000 

FREQUENCY IN CYaES 

Figure 66. Audibility of heterodyned noise 
from a 12-knot transport, masked by shrimp 
noise; arrows show limits of the band-pass filter. 
RD for presentation band shown: — 4 decibels. 
Character of primaudible signal : rhythm and 
change of quality. Primaudible frequency: 300 
cycles. See Table 5, line 28. (Matched.) 

signed to eliminate the “danger of the signal 
spectrum protruding beyond the edges of the 
noise spectrum, and consequently out of the 
masked region.”^ 

The spectra give average values of rms levels 
in 50-cycle bands at the various frequencies; 
the legends in Figures 66 through 75 refer to 
the corresponding data listed in Table 5. To 
distinguish tests performed with signal and 
noise pairs recorded through the same and 
through different types of gear, the latter have 
been marked with an asterisk in the table and 
so identified in the figure captions, but, in view 
of the difficulty of precisely duplicating record- 
ing conditions, it is not known how closely the 
“matched” sounds approximate the ideal of 
reaching the ear over the same hydrophone. 

From Table 5, the mean RD for the 14 
“matched” sounds is —2.9 decibels, with an 
average deviation from the mean of 1.2 deci- 
bels. These recognition differentials apply to 
the entire presentation band shown in the fig- 
ures. For the same group of sounds the aver- 
age signal-to-noise ratio, in the optimal 50- 
cycle band and at primaudibility, was —1.1 
decibels, with an average deviation from the 


V ItBS'lKlCTEU J 


UCDWR TESTS 


119 


Table 5. Primaudibilitv of supersonic sounds. 





Character 

of 

primaudible 

signal 

Recog- 

Signal-to-noise 
ratio in db in a 
O.l-lO kc band 

Signal-to-noise 
ratio in db in the 
optimal 50-cycle 
band at 
primaudibility 

Midfre- 

quency 

Signal 

Figure 

Merit 

of 

signal 

nition 
frequency 
region in 

80 

per cent 

50 

per cent 

in cycles 
of opti- 
mal 50- 



cycles 

recog- 

nition 

prob- 

ability 

recog- 

nition 

prob- 

ability 

Aver- 

age 

ratio 

Ratio for 
rhythm 
peaks 

cycle 

band 


A. Background : recorded submarine self -noise. 


1. Destroyer Escort 


Fair 

Rough rhythm 


-3 

-5 

-1 


550 

2. Twin screws of medi- 

71 

Good 

Rhythmic chug 


—1 

-4 

-3 


600 

um-size antisubmarine 
vessel at 13 knots 










3. Twin screws of medi- 


Good 

Rhythmic 


-1 

-3 

-3 


700 

um-size antisubmarine 
vessel at 5 knots 










4. Aircraft carrier at 12 


Fair* 

Twin screw 


0 

1 

2 


700 

knots 



rhythm 







5. Small Coast Guard 

68 

Fair* 

Rapid rhythm 


-3 

-3 

-5 


300 

patrol boat, at 10 
knots 










6. E.W. Scripps (single 


Fair* 

Flutter 


-2 

-4 

2 


300 

screw) at 7 knots 










7. Medium size trans- 


Fair* 

Single screw 


-2 

-5 

0 


300 

port at 12 knots 



rhythm 







8. PC at 15 knots 


Fair* 

Rhythmic 


-2 

-4 

1 


100 


B. Background: 

recorded self -noise on patrol craft, at 10 knots. 



9. Single screw of E. W. 


Fair 

Flutter 


1 

-2 

-1 


520 

Scripps at 7 knots 

10. Screws of S-class 


Fair* 

Rhythm 


-5 

—7 

-5 


900 

submarine at 6 knots 
and periscope depth 










11. PC at 15 knots 

73 

Fair 

Rhythm 


—1 

-2 

-2 


450 


C. Background: recorded self -noise on patrol craft, at 15 knots. 


12. Coast Guard cutter 


Fair* 

Rhythm 


-3 

- 5 

-1 


1,000 

13. Small auxiliary 


Fair* 

Flutter 


—4 

- 6 

-2 


1,500 

14. Aircraft carrier at 12 
knots 


Fair* 

Twin-screw 

rhythm 


-1 

- 3 

2 


1,000 

15. Small Coast Guard 
patrol boat at 10 knots 


Fair* 

Flutter 


-6 

- 7 

1 


1,000 

IQ. E. W. Scripps (single 
screw) at 7 knots 

70 

Fair* 

Rapid flutter 

.... 

-8 

-10 

2 


1,000 

17. Medium-size trans- 
port at 12 knots 


Fair* 

Single-screw 

rhythm 


-5 

- 6 

3 


1,000 

18. Screws of S-class 
submarine at 6 knots 
and 90-foot depth 

75 

Fair* 

Flutter 


-1 

- 3 

0 


1,500 

19. E. W. Scripps (single 
screw) at 7 knots 


Fair 

Flutter 


-1 

- 3 

—1 


1,500 

20. Screws of S-class 
submarine at 6 knots 
and periscope depth 

74 

Fair* 

Rhythm 


-5 

- 6 

-2 


1,500 


Signal and noise were recorded through different types of gear; therefore the RD is unreliable. 


RESTKTCTED 


120 


LARORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


Table 5. {Continued) 





Character 

Recog- 

Signal-to-noise 
ratio in db in a 
0.1-10 kc band 

Signal-to-noise 
ratio in db in the 
optimal 50-cycle 
band at 
primaudibility 

Midfre- 

quency 

Signal 

Figure 

Merit 

of 

signal 

of 

primaudible 

signal 

nition 
frequency 
region in 
cycles 

80 

per cent 
recog- 
nition 
prob- 
ability 

50 

per cent 
recog- 
nition 
prob- 
ability 

in cycles 
of opti- 
mal 50- 
cycle 
band 





Aver- 

age 

ratio 

Ratio for 
rhythm 
peaks 


D. Background: recorded deep-sea noise. Merit: good. 


21. Aircraft carrier at 

67 

Fair 

Twin-screw 


1 

0 

0 


1,600 

12 knots 



rhythm 







22. Small Coast Guard 


Fail- 

Flutter 


-1 

-3 

0 


300 

patrol boat at 10 knots 
23. E.W . Scripps (single 


Fair 

Rapid flutter 


-2 

-4 

1 


300 

screw) at 7 knots 










24. Medium-size trans- 


Fail- 

Single-screw 


-2 

-4 

1 


300 

port at 12 knots 



rhythm 







25. Aircraft carrier at 


Fair 

Twin-screw 


2 

0 

-2 


600 

12 knots 



rhythm 







26. Small Coast Guard 


Fail- 

Flutter 


. -2 

-4 

-3 


500 

patrol boat at 10 knots 
27. E.W. Scripps (single 


Fair 

Rapid flutter 


-1 

-2 

0 


1,000 

screw) at 7 knots 










28. Medium-size trans- 

66 

Fair 

Single-screw 


-3 

-4 

1 

-2 1 .... 

300 

port at 12 knots 



rhythm 

1 



1 

1 



mean of 1.1 decibels. Abnormally low recogni- 
tion differentials were observed for some of 
the unmatched sounds, but the critical-band 
criterion was found to apply in such cases (see 
Figure 70) . 

These values tend to confirm the opinion of 
experienced listeners that supersonic signals 
have little character and are more difficult to 
detect than sonic signals; the typical cue is a 
rhythmic time pattern in the primaudible sig- 
nal, as shown by the fourth column in Table 5. 
The tabulated descriptions of the primaudible 
signals indicate a wide variety of modulation 
types, but there is no corresponding variation 
in the primaudible signal-to-noise ratios; ab- 
sence of such a correlation is discussed below. 
The frequencies of primaudible components 
were not identified with the aid of band-pass 
filters. From the general parallelism of the 
signal and noise spectra, it would be antici- 


pated that in most cases the primaudible sig- 
nals were sensed as wide-band sounds covering 
essentially the same frequency range as the 
noise. Exceptions would be expected in cases 
like those illustrated in Figures 70 and 75, 
where the spectra were not matched. 

The limited information now available is 
inconclusive, but it seems to indicate that sonic 
propeller sounds are easier to detect than su- 
personic (see Figure 55, for example) and 
that the detectability of supersonic signals, in 
contrast to that of the sonic, is not correlated 
with the character of the signal modulation. 
It is impossible to decide, without further ex- 
perimental study, whether these differences are 
real or accidental. A possible defect of the 
supersonic signals which were used, namely 
“clipping” of the modulation peaks during re- 
cording, has already been mentioned ; also, 
modulation characteristics may be somewhat 


j RESTRICTED_^ j 


UCDWR TESTS 


121 


different at sonic and supersonic frequencies 
(see Figure 72 in this chapter and Figures 1 
and 11 in Chapter 3). On the other hand, it 
is possible that supersonic listening techniques 
can be improved in a number of ways. Thus, 



4661 2 4681 2 4681 

too 1000 10,000 

FREQUENCY IN CYCLES 

Figure 67. Audibility of heterodyned noise from 
a 12-knot aircraft carrier, masked by recorded 
deep-sea noise; arrows show limits of the band- 
pass filter. RD for presentation band shown; 

0 decibels. Character of primaudible signal : 
rhythm and quality change. Primaudible fre- 
quency: 1.6 kilocycles. See Table 5, line 21. 
(Matched.) 


sounds heterodyned from a narrow band cen- 
tered at 24 kilocycles involves a number of dis- 
advantages. High selectivity may render sig- 
nal modulations less audible. Similarly, the 
signal envelope may have unfavorable charac- 



4681 2 4681 2 4681 

100 1000 10,000 

FREQUENCY IN CYCLES 

Figure 68. Audibility of heterodyned noise from 
a small 10-knot patrol craft, masked by self- 
noise recorded aboard a submarine proceeding at 
3 knots and a depth of 90 feet. RD for presen- 
tation band shown; — 5 decibels. Character of 
primaudible signal: rapid rhythm. Primaudible 
frequency: 300 cycles. See Table 5, line 5. (Not 
matched.) 


the widest audio band commonly available does teristics at the lower beat frequencies ; the 

not exceed 2 to 3 kilocycles, because supersonic heterodyned band may be folded, which is not 

listening is currently done with receivers de- necessarily an advantage, and the intensity 
signed for echo ranging. Inasmuch as the echo- limen is less favorable for the low audio fre- 

ranging signal is usually a tonal pulse, it is quencies (Figure 15 in Chapter 2) . These con- 


o 

'!C 

z 



2 

1 

0 


.8 UO 1.2 1.4 

TIME IN SECONDS 


Figure 69. Time-frequency-intensity analysis for the self-noise shown in Figure 68. 


convenient to restrict the band width of the 
noise background so that the listening level for 
the primaudible frequencies may be raised to 
an optimal value without discomfort. On the 
other hand, listening to supersonic propeller 


siderations suggest that better supersonic lis- 
tening might be done with a nonfolded band 
presented between about 0.5 and 8 kilocycles. 

That some such improvement may be possible 
is indicated by quasi-held observations^^ which 



122 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


show that the spectrum differential for sonic components in the sonic ship sisals was 
and supersonic propeller sounds alike is of the threshold-limited, or masking-limited (see Sec- 
order of —6 decibels, in agreement with the tion 3.4.1). 



4681 2 4681 2 4681 

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Figure 70. Audibility of heterodyned noise from 
a 7-knot schooner, masked by self-noise recorded 
aboard a 15-knot patrol craft at a projector 
bearing .of 090; arrows show the limits of the 
band-pass filter. RD for presentation band 
shown: — 10 decibels. Character of primaudible 
signal: rapid flutter. Primaudible frequency: 

1 kilocycle. See Table 5, line 16. (Not matched.) 

theory developed in Section 4.2.3. These tests 
were performed with simulated propeller 
sounds obtained from wide band thermal noise 
which was 50 per cent amplitude modulated. 



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FREQUENCY IN CYCLES 

Figure 71. Audibility of heterodyned noise from 
a 13-knot Coast Guard cutter, masked by self- 
noise recorded aboard an S-class submarine pro- 
ceeding at 3 knots and a depth of 90 feet; the 
arrow shows the limit of the low-pass filter. RD 
for presentation band shown: — 4 decibels. 
Character of primaudible signal: rhythmic chug. 
Primaudible frequency: 600 cycles. See Table 5, 
line 2. (Matched.) 

The pure-tone threshold was determined un- 
der quiet conditions for two observers wearing 
headphones and is shown in Figures 76, 77, 78, 
and 79. When this threshold is equated to the 



Figure 72. Time-frequency-intensity analysis for the self-noise from a Coast Guard cutter (Fig- 
ure 71). 


Masking of Pure Tones 

Recognition tests with pure tones were con- 
ducted to study the application of the critical 
band criterion to backgrounds other than those 
used in the original work (see Chapter 2) and 
to determine whether the audibility of tonal 


free-field threshold (Figure 1 in Chapter 2) for 
any value above 1 kilocycle, the two sets of de- 
terminations are found to be in good agree- 
ment between 1 and 8 kilocycles, which is the 
highest frequency shown in Figures 76 through 
79, and to diverge progressively below 1 kilo- 
cycle, so that the free-field threshold lies more 




RESTRICTE 


UCDWR TESTS 


123 


than 10 decibels below the headphone threshold threshold differs from that shown in Chapter 2, 
at 150 cycles. This inferiority of the headphone since it was measured in terms of voltage 
threshold at the low frequencies appears to be applied rather than output pressure. 



4681 2 4681 2 

100 1000 


4 6 8 1 

10,000 


FREQUENCY IN CYCLES 


Figure 73. Audibility of heterodyned noise from 
a 15-knot patrol craft, masked by self-noise 
recorded aboard a 10-knot patrol craft at a pro- 
jector bearing of 090; the arrow shows the limit 
of the low-pass filter. RD for presentation band 
shown: — 2 decibels. Character of primaudible 
signal: rhythmic noise. Primaudible frequency: 
450 cycles. See Table 5, line 11. (Matched.) 



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Figure 74. Audibility of heterodyned noise from 
an S-class submarine proceeding at 6 knots and 
periscope depth, masked by self-noise recorded 
aboard a 15-knot patrol craft at a projector 
bearing of 090 ; the arrow shows the limit of the 
low-pass filter. RD for presentation band shown: 
— 6 decibels. Character of primaudible signal: 
changed rhythm. Primaudible frequency: 1.5 
kilocycles. See Table 5, line 20. (Not matched.) 



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FREQUENCY INCYCLES 

Figure 75. Audibility of heterodyned noise from 
an S-class submarine proceeding at 6 knots and 
a depth of 90 feet, masked by self-noise recorded 
aboard a 15-knot patrol craft at a projector 
bearing of 090; the arrow shows the limit of the 
low-pass filter. RD for presentation band shown: 
— 3 decibels. Character of primaudible signal: 
roar. Primaudible frequency: 1.5 kilocycles. See 
Table 5, line 18. (Not matched.) 



FREQUENCY IN CYCLES 

Figure 76. Audibility of pure tones, masked by 
simulated deep-sea noise. The solid line repre- 
sents background measurements made with a 
50-cycle filter; the dashed line represents com- 
puted levels of primaudible tones. Circles and 
triangles represent primaudible levels of tones 
determined by two observers. 


typical (see Figures 76. through 79 and also 
Figure 4 in Chapter 10). The character at the 
low frequencies of the present headphone 


Four widely different types of distributed 
noise, selected from the sounds used in the 
masking tests and presented over the full 0.1- 


124 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


to 10-kilocycle band at a listening level of about 
60 phons, 'were used as backgrounds. The 50- 
cycle spectra of these sounds are shown as solid 
lines in Figures 76 through 79; and critical 
band spectra, computed from the 50-cycle spec- 
tra by adding the quantity 10 log (critical band 



Figure 77. Audibility of pure tones, masked by 
shrimp noise. The solid line represents back- 
ground measurements made with a 50-cycle 
filter; dashed line represents computed levels of 
primaudible tones. Circles and triangles repre- 
sent primaudible levels of tones determined by 
two observers. 

width/50), obtainable from Figure 17 in Chap- 
ter 2, are represented by broken lines. The 
two sets of spectra do not differ by more than 
a few decibels from 0.1 to 2.5 kilocycles. 

The levels of primaudible tones at various 
frequencies are given as individual points, and 
these tone levels coincide fairly well with the 
critical band spectra. The performance of the 
two observers differs by 2 to 3 decibels, which 
is within the probable limits of individual vari- 
ation. Error in measurement of gain setting 
is the simplest explanation of the apparent 
audibility of a 125-cycle tone below threshold 
(see Figure 78), and may also account for the 
negative tendency of the points below 1 kilo- 
cycle, which appears in this figure only. It is 
possible, however, that poor attenuation of 
room noise at the low frequencies raised the 
observed threshold in this region, and that air- 
borne noise fell to a lower level during the 
subsequent masking tests. Changes in adjust- 


ment of the headphones may have similar ef- 
fects. The 125-cycle points in Figures 77 and 
79 lie 10 to 15 decibels above the critical band 
spectra because the level of background at this 
frequency is below the threshold of hearing; 
in other words, for optimal results, the back- 
ground should be audible throughout the lis- 
tening band. In general, therefore, the RD 
for a given pair of sounds will be approxi- 
mately independent of gain when the signal 
is masked by noise received through the hydro- 
phone. However, the RD will change with gain 
setting when signal audibility is either thresh- 
old-limited or limited by airborne noise. 

The requirement that background be audible 
throughout the listening band may be difficult 
to meet if, as in the case of the shrimp back- 
ground illustrated in Figure 35, the high-fre- 
quency noise components make the major con- 
tribution to the total loudness. It is possible 
that the deviations from the critical-band rule 


o t 









































“X — 

A 










— N 


O 
















\\ 












\\ 

\ \ 












\ A 













'\ 

^ \ 




Of 

3SERVE0 THRESHOLD 

1 c-i/e-i cno Tr^Mce 

\ 




V 








N 


\ 













> 



























2 4 6 6 1 2 

1000 

frequency in cycles 


6 8 I 
10,000 


Figure 78. Audibility of pure tones, masked by 
heterodyned supersonic noise. The solid line 
represents background measurements made with 
a 50-cycle filter; the dashed line represents com- 
puted levels of primaudible tones. Circles and 
triangles represent primaudible levels of tones 
determined by two observers. 


in the case of shrimp-noise backgrounds, as dis- 
cussed in Section 4.2.2, were brought about by 
threshold-limited listening, since the back- 
ground noise in those tests was also presented 
at a level of about 60 phons. However, the lis- 
teningj^els in the present and the previously 


UCDWR TESTS 


125 


discussed tests were only approximately equal, 
and in the latter case were subject to some con- 
trol by the observers ; hence, no definite conclu- 
sion can be drawn. 

On the other hand, general use of low-pass 
filters in the presence of shrimp noise is inad- 
visable because optimal signal components at 


Ui < 




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100 1000 10,000 
FREQUENCY IN CYCLES 


Figure 79. Audibility of pure tones, masked by 
flat thermal noise. The solid line represents back- 
ground measurements made with a 50-cycle 
Alter; the dashed line represents computed levels 
of primaudible tones. Circles and triangles 
represent primaudible levels of tones determined 
by two observers. 


frequencies beyond the low-pass limit may be 
rendered inaudible. One solution would be to 
design the response of the entire system, in- 
cluding phones or speaker, so that the critical- 
band spectrum of the noise background, mea- 
sured at the ear, parallels the threshold curve. 
For a hydrophone directional in one dimension 
(such as the JP-1), the slopes of ambient and 
of self-noise spectra are about 9 and 12 deci- 
bels per octave, respectively; therefore, two 
tilting networks would suffice to assure equally 
good conditions at all listening frequencies and 
on most occasions. Presentation like that illus- 
trated in Figure 76 is preferable to that shown 
in Figures 77 and 79, since it interferes less 
with the audibility of low-frequency compo- 
nents in the signal. 

The background shown in Figure 78 was 
obtained from a supersonic receiver, and the 
slope of —20 decibels per octave in the region 
above 2 kilocycles represents the suppression 


characteristic of a filter. The fact that the 
critical band criterion is obeyed at 4 kilocycles 
implies that remote masking is relatively un- 
important for masking backgrounds with con- 
tinuous spectra whose slope is less than —20 
decibels per octave (see also Figure 6 in Chap- 
ter 2), in other words, that in such cases the 
masking is produced primarily by background 
components included within the various criti- 
cal bands and that components lying outside 
any critical band make a negligible contribu- 
tion to the masking within it. It seems prob- 
able, therefore, that the critical-band criterion 
also applies when the background has a slope 
as great as +20 decibels per octave, since re- 
mote masking is less effective in the direction 
of low frequencies. Finally, it may be noted 
that differences in time pattern of the masking 
backgrounds used in the pure-tone tests seem 
to have had little effect on the results. 


Subjective Aspects 

“In general it may be stated that learning 
effects were moderate in magnitude and the 
learning period was short. Fatigue was virtu- 
ally unobserved, even for continuous listening 
over a 2-hour period, although this observation 
is not applicable to shipboard operation where 
the environment is much less ideal. In gen- 
eral the experienced listeners showed a sharper 
transition from ‘heard' to ‘not heard’ than 
inexperienced listeners who often had a ten- 
dency to report ‘heard’ when no signal was 
present. The Sound School graduates with the 
highest grade on the doppler drill appeared to 
show a sharper transition [from ‘not heard’ to 
‘heard’] than others but they did not appear 
able to hear a [masked] signal at a lower 
[relative] level than others.”^ 

The doppler drill, which is a standard test 
used in selection of sonar personnel, measures 
ability to discriminate the pitch of a test 
tone from that of a reference tone. The tones 
are presented in succession and without inter- 
ference from masking sounds. Absence of cor- 
relation between performance in the doppler 
drill and the masking tests is surprising in 
view of the close relation between frequency 


STRTCTED 


126 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


discrimination and critical band width (see 
Sections 2.2.1 and 2.3) . This point may deserve 
further study; for example, the observer who 
gave consistently better-than-average perform- 
ance in the pure-tone tests (Figures 76 through 
79) was a trained musician. 

A set of tests conducted in connection with 
the proposed masking monitor provides a use- 
ful index to the typical spread among recogni- 
tion differentials for individual listeners. Mask- 
ing tests were made in the laboratory using a 


at both conditions were recorded and the mean 
taken as the recognition value. 

'Tn the first test, the determinations were 
made without interrupting the submarine 
sound. In the second test, the subjects were 
instructed to interrupt the submarine sound at 
will, to [ascertain] whether such technique 
facilitated determinations.’' The results are 
shown in Table 6. 

All levels in Table 6 are stated in decibels 
relative to a single arbitrary reference, and 


Table 6. Effect of experience and signal modulation. 



Signal not interrupted 
Masking-noise level in db 

Signal interrupted 
Masking-noise level in db 


Signal 

audible; 

noise 

decreasing 

Mean value 

Signal 

masked; 

noise 

increasing 

Signal 

audible; 

noise 

decreasing 

Mean value 

Signal 

masked; 

noise 

increasing 

3 different experienced observers 

12 

12.5 

13 

15 

15.5 

16 


14 

15 

16 

16 

16.5 

17 


14 

15 

16 

16 

18 

20 

Average in db 


14.2 



16.7 


Mean deviation in db 


1.1 



0.9 


6 different inexperienced observers 

6 

8.5 

11 

16 

17 

18 


9 

9.5 

10 

14 

14.5 

15 


12 

13.5 

15 

18 

18.5 

19 


13 

14 

15 

15 

16 

17 


10 

11 

12 

15 

16 

17 


12 

14 

16 

14 

15.5 

17 

Average in db 


11.8 



16.2 


Mean deviation in db 

Difference in db between best and 


2.1 



1.0 


poorest observer 


6.5 



4.0 



recording of a fleet-type submarine at periscope 
depth and a speed of 3 knots. Nine listeners 
participated ; of these, only three had previous 
experience in masking tests. In this test, the 
listeners could control the level of the masking 
background, which was wide-band thermal 
noise, and the gain setting in the signal chan- 
nel was fixed at a comfortable listening level. 
''All [observers] were given the same instruc- 
tions,” according to an informal communica- 
tion from the University of California Division 
of War Research [UCDWR], "to turn the noise 
dial until the submarine sound was just masked 
by the noise, then to back it off until the sound 
was again audible. The noise attenuator values 


the higher noise levels correspond to better 
performance; thus the table shows relative, 
not absolute performance. The method of mea- 
suring performance in this case was essentially 
that of changing the nature of the stimulus 
in a progressive series of small steps which 
traversed, and thereby defined, the region of 
the response threshold. It is known as the 
method of minimal increments and has the 
advantage of being less time consuming than 
the random-order method. The method of in- 
crements defines the borders of the transition 
zone but gives no clear picture of the transi- 
tion curve. It is commonly observed in such 
tests that the observers’ thresholds are biased, 

CTW 


UCDWR TESTS 


127 


in the sense that performance is better when 
the stimulus is changed from “heard” to “not 
heard” (labeled “noise increasing” in Table 
6) than when the reverse change is made 
(“noise decreasing”). This is often attributed 
to a “memory effect”; in other words, if the 
cue is definite at the outset and is gradually 
faded out while the observer continues to lis- 
ten, he can usually follow it a little further be- 
low noise than when the cue is not sensed in 
the beginning and subsequently emerges into 
consciousness. This type of response probably 
depends more on continuity of sensation than 
it does on knowing and remembering the char- 
acter of the primaudible cue. In practice, there- 
fore, it is generally easier to maintain contact 
with a target than to make first contact. 

The mean of the ascending and descending 
threshold determinations is usually taken as 
the minimal-increment RD (for an individual 
or a group) ; and the mean RD in the UCDWR 
tests was usually about 2 decibels larger (less 
favorable) than when the determination was 
made by the random-order method. Stated dif- 
ferently, a minimal-increment determination 
corresponds approximately to the 75 per cent 
point on a recognition probability curve, since 
an increase of about 2 decibels will, on the 
average, raise the recognition probability from 
50 to 75 per cent (see equation (4) in Section 
4.1.4). 

It has been noted in Section 4.1.5 that the 
opposite result was apparently obtained by the 
British, who found that the minimum-incre- 
ment recognition differentials were frequently 
less (more favorable) than the random-order 
recognition differentials, some differences 
amounting to as much as 5 to 6 decibels. This 
finding may be attributed to differences in in- 
doctrination used in the two sets of tests. In 
the British random-order tests, the observers 
were not trained to listen consciously to sounds 
of all frequencies; in the random-order tests 
they presumably concentrated on that region 
of the spectrum where the target sound was 
most audible in the absence of the masking 
background, while in the increasing-level tests 
they were forced by necessity to listen to all 
frequencies, since the target sound had not yet 
been heard. The UCDWR observers were 


trained in all cases to scan the frequency spec- 
trum and were warned that the most prominent 
features of the signal at a high sensation level 
might not be the cues at primaudibility. 

Table 6 also reveals the effectiveness of audi- 
tory motion, whether introduced by modula- 
tion of the signal or by training a directional 
hydrophone. Thus, interrupting the signal im- 
proved the average performance of the experi- 
enced group by 2.5 decibels and that of the 
inexperienced group by 4.4 decibels. In other 
words, when rapid comparisons can be made 
between pure background and the signal-back- 
ground mixture, the minimal-increment RD 
corresponds roughly to 50 per cent, rather than 
to 75 per cent recognition probability. It will 
be noted, incidentally, that signal interruption 
affects both ascending and descending recog- 
nition differentials, that is, the entire transi- 
tion curve is shifted. The communication de- 
scribing these results observes that “although 
some of this improvement might have resulted 
from experience gained in (the first test), the 
comments of the observers indicated a greater 
sense of reliability for the signal-interruption 
type of test.” 

The relative performance of the two groups 
was nearly identical under the second condi- 
tion of test, but even in this case, the best 
observer outperformed the poorest by 4 deci- 
bels. The practical situation is more compli- 
cated than the one used in this test ; hence, the 
effects of training and aptitude will usually 
be larger. Table 6 is in agreement with pre- 
vious indications that signal modulation may 
help by 2 to 3 decibels (see Section 4.2.2) ; 
possibly this factor obscured such differences 
in performance as would be expected purely 
on the basis of grades in the doppler drill. 

For experienced observers, the average devi- 
ation from the mean performance in the mini- 
mal increment tests was about 1 decibel. This 
was also true of the various random-order 
tests, except for cases in which different com- 
ponents in the signal were detected by different 
listeners. Under these circumstances, discrep- 
ancies as great as 8 decibels occurred (see also 
Section 4.1.5). In cases like that illustrated in 
Figure 34, some observers reported the signal 


RESTRICTED 


128 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


primaudible at 120 cycles; some, at 1,400 
cycles; and others, at both frequencies. 

In general, it was found that judgments were 
easiest and most consistent when a hydrophone 
sweep was simulated by fading the signal into 
background and out again. The random-order 
method was of intermediate difficulty, since it 
requires that a mental image of the pure back- 
ground noise be retained. The judgments were 
most difficult to make and the response least 
consistent when the observers listened to the 
background noise continuously for a 2-hour 
period during which various signals were 
slowly brought into audibility at random inter- 
vals (separated by about 10 minutes, on the 
average) and faded out again. Detection under 
these conditions required, on the average, about 
2 to 3 decibels more gain in the signal channel 
than was needed for 50 per cent detection of 
the same signals by the random-order method, 
which corresponds roughly to the 90 per cent 
point on the transition curves. Auditory fatigue 
was apparently not an important factor in 
these tests, since the observers performed as 
well at the end of the two hours as at the be- 
ginning. The 2-hour test resembled shipboard 
listening in some respects, but it gave no 
weight to the help which would probably be 
derived from training a directional hydro- 
phone. For practical purposes, therefore, the 
50 per cent RD determined by the random- 
order method is probably as useful an index 
of detectability as any. 


CUDWR-NLL TESTS 

In addition to the field tests described in 
Chapter 5, some laboratory tests on masking 
of sounds were also carried out at New London. 
These tests were roughly similar in technique 
to those described in previous sections of this 
chapter. Their objectives were somewhat less 
general, however, in that they were designed 
to give specific answers to certain questions 
which arose in the design of sonic listening 
gear. These tests are described in the next three 
sections. 


^ ^ Interval Tests 

A study^^ was undertaken which was de- 
signed to determine whether “an optimum 
length of listening interval exists when switch- 
ing from one hydrophone to another in a series 
of cable-connected hydrophones or from one 
buoy to another in the case of radiosonic 
buoys.’’ To this end, masking tests were made 
in which various signal-background mixtures 
were presented to the observers for selected 
lengths of time. As shown in Figure 81, per- 
formance was significantly influenced by the 
length of the listening interval. 

Four test series were run, most of them in 
duplicate. In each series, 25 pairs of sounds 
were presented according to the time schedule 
shown in Figure 80, where A and C represent 


ABC 
SOUND SILENCE SOUND 



Figure 80. Sequence within a test pair, where 
time intervals U, U, and tz are equal. 


members of such a test pair, each member 
being equal in duration to the intervening 
period of silence denoted by B, When the first 
of the three intervals shown in Figure 80 con- 
sisted of a mixture of signal and background, 
the third contained only background, and vice 
versa. The order in which the mixture and the 
pure background occurred in successive pairs, 
as well as the signal-to-background ratio for 
the mixture, was randomized, but the lengths 
of the equal time intervals (^i, t^) were 

restricted in each sequence of 25 pairs of 
sounds to 2, 3, 4.5, or 7 seconds. Between suc- 
cessive pairs in a test sequence of 25, the ob- 
servers indicated on a score sheet whether, in 
their judgments, the signal had occurred in in- 
terval A or interval C of the immediately pre- 
ceding test pair. 

Since the experimental variable of interest 
was the length of listening interval, the same 
signal and background were used throughout. 
The former was a recording of the underwater 
sonic output of a freighter and contained a 
typical assortment of thumps, engine noises, 
and propeller sounds ; the masking background 


hestricte: 


CUDWR-NLL TESTS 


129 


was water noise, extending over a wide fre- 
quency band. The apparatus used was essen- 
tially that illustrated in Figure 1, except that a 
wide-band high-fidelity loudspeaker was substi- 
tuted for the headphones. The gain in the back- 
ground channel was set and maintained 
throughout the test at a fixed level which the 
listeners found comfortable. The signal-to- 
background ratio for successive pairs was va- 
ried at random over the range shown in Figure 
81, which was covered in 5-decibel steps as 
shown by the experimental points. Since there 
were 11 observers and duplicate tests were 
made for all but one of the signal-to-noise 
ratios, the experimental points corresponding 
to —5 decibels represent 55 independent judg- 



-5 -0 -5 -10 -15 

SIGNAL TO NOISE RATIO IN OB ABOVE MINI>4AL INCREMENT RQ 


Figure 81. Effect of presentation interval on 
detection, where numerals indicate durations of 
presentation intervals in seconds. 


member of a test pair; since the subjects are 
required in this type of test to make some as- 
signment for each pair of sounds, they should 
in general receive a score of 50 per cent for 
signal-to-noise ratios at which the probability 
of perception is vanishingly small. In other 
words, a score of 50 per cent corresponds to 
inability to make the required discrimination, 
or 0 per cent perception. Similarly, a score of 
75 per cent in this type of test is usually con- 
sidered equivalent to a 50 per cent probability 
of perception. Setting up these correspondences 
between p per cent on the curve of perceptions 
and c per cent on the curve of correct answers 
may be expressed quantitatively in the follow- 
ing way. The quantity (100 — p) per cent rep- 
resents the ‘‘recognition impairment,” the rela- 
tive number of presentations of a given kind 
in which the observer fails to make the desired 
discrimination. If he guesses in these cases, 
he will in the long run make the correct guess 
half the time. Hence, the apparent impair- 
ment (100 — c) per cent will be only half as 
great as the true impairment (100— p) per 
cent. Thus, (100 — c)/(100— p) =%, and c = 
(100+p)/2. Substituting in this expression 
shows that, as already stated, the following sets 
of values of p and c are equivalent, and similarly 
at intermediate points : 

p per cent 100 80 50 20 0 

c per cent 100 90 75 60 50 


ments; all other points in Figure 81 represent 
110 judgments. The signal-to-noise ratio labeled 
0 decibels in this figure represents the value 
which experienced observers designated as 
primaudible. This determination of primaudi- 
bility was made prior to the interval tests by 
the method of minimal increments, and no time 
limitation was imposed on the listeners. The 
various signal-to-noise ratios specified in Fig- 
ure 81 are referred to this independently de- 
termined minimum increment RD. 

In essence, the four sets of points which 
have been connected by curves in Figure 81 
are points defining transition curves. These 
transition curves differ in a number of ways 
from the ones given in Figures 6 through 23. 
For example, in the present case the ordinate 
indicates primarily the relative number of suc- 
cessful assignments of the signal to the proper 


From equation (4) in Section 4.1.4 it will be 
seen that the half-spread, or decibel increase 
in the signal-to-noise ratio which improves the 
probability of perception from 50 per cent to 
80 per cent is 6/n decibels. The preceding tabu- 
lation shows that an equivalent number of 
decibels separates the 75 and 90 per cent points 
on the curve of correct responses. The interval 
between 75 and 90 per cent averages 4 decibels 
for the curves in Figure 81 whence, n=l.^ 
decibels. Since a decrease of G/ti decibels pro- 
duces a 30 per cent impairment in the proba- 
bility of perception, the ratio (6/7t)/30 states 
the number of decibels required to produce a 1 
per cent impairment (over the nearly linear 
portion of the transition curve between the 20 
and 80 per cent points). When ti=1.5, the 
decibel loss which leads to a 1 per cent impair- 
ment is nearly twice as large as in the typical 


IRICTED^l/ 


130 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


case (where n is about 2.5). This is due, pre- 
sumably, to the fact that observers usually fail 
to report the doubtful perceptions when guess- 
ing is penalized, as in the random-order tests 
discussed earlier in this chapter. In practice, 
therefore, where doubtful contacts are reported 
as a matter of course, the spreads of the rele- 
vant transition curves are probably somewhat 
larger than shown in Figures 6 through 23, 
and more nearly those in Figure 81. It should 
also be noted that the curves in Figure 81 do 
not approach a value of 50 per cent even for 
the smallest signal-to-noise ratio shown; that 
is, some discrimination is possible at ratios 
more than 5 decibels below the nominal thresh- 
old value, which in this case occurs at the 75 
per cent point (see also Figures 6 through 23). 
Similarly, it will be clear from Figure 81 why 
an interval of 2 decibels is usually considered 
necessary for precise definition of the tran- 
sition curve. 

It proves helpful to consider an additional 
difference between the procedure used in the 
tests described in Sections 4.1 and 4.2 and the 
procedure used in these interval tests. In the 
present case, the observer is able to make a 
more nearly immediate comparison between 
the mixture and the pure background than is 
possible in the former, and is, therefore, less 
dependent on a memory image of the 
primaudible cue. Such images fade rather 
quickly. Hence, the fact that two members of a 
test pair could be compared after a smaller 
time lapse for the shorter listening intervals 
(see Figure 80) may well have had more 
weight in determining the nature of the re- 
sults than did the durations of the test sounds 
themselves. Thus, it will be seen from Figure 
81 that in general — and aside from irregu- 
larities like the one in the curve relating to the 
3-second interval (which irregularities could 
probably be minimized by making a larger 
number of determinations and varying the sig- 
nal-to-noise ratio in smaller steps) — the trend 
associated with a decrease in the time lapse 
between members of a test pair is an increased 
ability to perceive the signal. This improve- 
ment would be expected on the basis of the 
discussion in Sections 5.1 and 5.6. It may be 
noted that the 75 per cent point on the curve 
for a 7-second interval (an interval between 


successive presentations which is nearly equal 
to that employed in the previously described 
test procedures) falls on the 0-decibel line, 
which represents the minimal increment RD 
for experienced observers, whereas the 75 per 
cent points for shorter time intervals occur at 
smaller signal-to-noise ratios. 

In assessing the difference of nearly 5 
decibels between the 75 per cent points of the 
curves for 2 and 7 seconds, it should be noted 
that impairment of perception due to unfavor- 
able coincidences between signal and noise 
peaks is probably not very significant for listen- 
ing intervals more than 1 second in duration 
(see Section 8.4.4). Operation of this factor 
furnishes an additional reason for sweeping 
the axis of a directional hydrophone several 
times in succession across a suspected bearing 
in order to obtain the maximum advantage 
from the locally produced modulation. On the 
other hand, the rapid pace and higher degree 
of concentration required when the listening 
interval was reduced to 2 seconds was con- 
sidered too fatiguing by the observers, who 
agreed that periods of active listening should 
probably be no less than about 3 to 4 seconds 
during a watch of practical length. Hence, the 
presentation interval of 5 to 10 seconds nor- 
mally used in listening tests is probably ade- 
quate so far as reduction of fatigue effects is 
concerned. Some observations made during 
British field tests of radiosonic equipment 
indicate that it is probably not useful to extend 
a watch beyond a half hour ; at the end of that 
time, listeners tended to become confused and 
to make increasingly numerous errors of com- 
mission. Finally, it should be pointed out that 
the 7-second interval was the one used in the 
first test of this series. Hence, if any learning 
occurred during the course of the tests it 
probably affected the results more for the 7- 
second interval than for the others; such an 
effect would make the observed improvement 
above performance in the 7-second test greater 
than the improvement actually due to dimin- 
ished time lapse. 

Peak Tests 

The response of hydrophones often shows 
one or more peaks, or maxima, that occur at 



CUDWR-NLL TESTS 


131 


frequencies to which the hydrophone structure 
resonates. When such resonances fall within 
the intended listening band, they may limit the 
usefulness of the receiver. If the resonance 
frequency, or frequencies, cannot conveniently 
be shifted beyond the confines of the listening 
band, it may still be possible to control their 
magnitudes by relatively simple means. Thus, 
it becomes desirable to know how high a re- 
sponse peak may be tolerated without signifi- 
cantly impairing performance. The tests de- 
scribed in this section^® were designed to deter- 
mine the permissible heights of resonance 
maxima in the response of hydrophones like 
the JP-1. The latter, a 3-foot nickel tube, may 
show a disturbing degree of longitudinal reso- 
nance (vibrations propagated along the axis 
of the cylinder) unless its ends are clamped be- 
tween rubber blocks so as to damp out this 
mode of vibration. 

It may be noted in passing that transfer of 
energy to the longitudinal mode of vibration is 
unavoidable since the application of stress 
along a given axis of a solid produces a de- 
formation not only in that direction but also 
along axes transverse to the first; the magni- 
tude of the secondary strain in terms of the 
primary is given by Poisson’s ratio. If the lon- 
gitudinal vibration of a tubular hydrophone 
supported along a transverse axis through its 
center resembles that in a rod clamped at its 
center, the fundamental mode of vibration in- 
volves a wave whose length A is twice that of 
the cylinder axis d; in other words, A = 2d, or 
f = c/2d, where / is the resonance frequency 
and c is the velocity of sound propagation 
(about 1.6 X 10^ feet per second in the case of 
nickel) . For a 3-foot nickel tube, the funda- 
mental frequency of the longitudinal resonance 
should therefore occur at about 2.5 kilocycles, 
which is within the sonic listening band. Al- 
though such a peak is not shown in Figure 2 
of Chapter 5, because the resonance frequency 
was not used during calibration, fairly strong 
maxima are often observed in the spectra of 
self-noise received in JP-1 gear. 

In practice, the hydrophone is immersed in 
a random noise field (this description applies 
to most signals as well as most backgrounds) ; 
hence, the resonance vibrations are shock- 
excited intermittently and persist, or decay. 


over an interval which depends on the ''Q,” or 
sharpness of tuning, of the resonance charac- 
teristic. Thus, when a broad-band sound is re- 
ceived over a resonant hydrophone, the listener 
hears a fiuctuating tonal ring (like that emitted 
by a public address system with a moderate 
amount of acoustic feedback) superposed on 
the sound proper. When the ringing sound is a 
prominent and fairly continuous component of 
the normally received background, the result- 
ing annoyance and distraction interfere with 
careful search and may lead the operator to 
drop the gain too far. If the optimal component 
received from a given target lies in a frequency 
region fairly close to that of the resonance peak, 
the signal may not become primaudible until 
the signal-to-noise ratio is significantly greater 
than would otherwise be required. This would 
not necessarily be true, however, especially if 
the time pattern of the resonance is different 
for the mixture and for pure background. For 
example, the resonance peak shown near 2 
kilocycles in Figure 77 apparently had very 
little effect on the audibility of the 2 kilocycle 
tone. 

The present test sheds no light on the mask- 
ing problem, but it does indicate the condition 
in which a resonance peak will be annoying. 
Since such peaks will not annoy unless they 
can be heard, the observers were tested on their 
ability to detect the presence of resonance 
peaks in several sequences of sounds. Two re- 
cordings were used: one obtained from the 
sonic underwater output of a small surface 
vessel, and the other, from that of a surfaced 
submarine. The recording hydrophone, system, 
and medium were fairly flat in frequency re- 
sponse and relatively free of resonances. Thus, 
the outputs from the recordings were wide- 
band sounds whose spectra had the charac- 
teristic negative slope of about — 6 decibels per 
octave. In order to simulate the rising response 
characteristic of the JP hydrophone (see Fig- 
ure 2 in Chapter 5), the electrical outputs from 
playbacks of the recordings were transmitted 
through a tilting network which increased the 
applied gain at the rate of about 6 decibels per 
octave (see Figure 82), so that the presented 
sounds had essentially flat spectra. This trans- 
mission network also contained controllable 
resonance elements by means of which it was 


I ^STRIGTE]^ 


132 


LABORATORY MEASUREMENTS ON MASKING OF TARGET SOUNDS 


possible to simulate the intermittent ringing 
note due to hydrophone resonance and to bring 
the resonance peak in at any of three different 
frequencies (0.9, 1.4, and 2.0 kilocycles), with 
one of six heights relative to the adjacent 
spectrum levels (the added gains were 0, 2, 4, 
6, 9, and 12 decibels), and with a Q factor of 
30, 35, or 50. One of these conditions is illus- 
trated in Figure 82 where the dotted portion 
of the curve applies to a peak height of 0 
decibels. 

The test sounds were presented to between 9 
and 11 observers by means of a high-fidelity 
loudspeaker and at a comfortable listening level. 
No sound mixtures were used, and the only 
distinction that the observers had to make was 
between presentations in which the resonance 
note was audible and those in which it was 
inaudible. Ten random-order tests were admin- 
istered, each consisting of 24 sound samples 
among which the six available peak heights 
were equally and randomly distributed ; samples 
were 5 seconds long and were separated by a 
silent interval of 3 seconds during which the 
listeners expressed their judgments on pre- 
pared score sheets. The ship sound recording, 
as well as the frequency and sharpness of the 
resonance peak, was fixed throughout each 
group of 24 samples, the only variable being 
peak height. Among the ten tests, however, 
different combinations of recording, Q, and 
peak frequency were tried. For each of these 
ten combinations, typical random-order tran- 
sition curves were obtained, showing the proba- 
bility of detecting the presence of the resonance 
note as a function of peak height. 

In most cases, a peak height of 6 decibels 
was recognized in 50 per cent of the trials, and 
a height of 9 decibels was detected in 80 per 
cent of the trials (in other words, n=2) . Two 
experienced sonar operators, who did not par- 
ticipate in the preceding tests, indicated that 
they found a peak of about 8 decibels annoying. 
The transition curves showed almost no de- 
pendence on the ship sound used or the fre- 
quency of the peak (for the three frequencies 
tried), but were significantly influenced by the 
Q of the resonance so that lower peaks were 
more readily detected when Q was large. 
Among other things, the duration of the inter- 


mittently excited ringing note is determined by 
the sharpness of tuning (see Section 7.1) ; for 
the tests described here, the durations of the 
tonal pulses which constituted the audible 
evidence of a resonant condition were of the 
order of 0.1 second (as indicated by the widths 
of the resonance peaks). In Section 2.2.2, it 
was shown that the subjective loudness of a 
tonal pulse, as well as its audibility in the 
presence of masking background, is less than 
that of a sustained tone of the same frequency. 
Figure 8 in Chapter 8 shows that the loss in 
audibility of a 100-millisecond pulse, relative 
to that of a sustained tone, is of the order of 7 
decibels, and that this loss is comparatively in- 
dependent of frequency over a large frequency 
interval. In other words, a mixture of pulse and 
noise background can be distinguished half the 
time from the pure background (contained in 
a critical band centered at the pulse frequency) 
when the pulse-to-noise ratio is about 7 decibels 
and the pulse length is about 100 milliseconds; 
essentially these conditions prevailed during 
the present tests on the audibility of resonance 
peaks. Thus, it may be inferred from the 
preceding observations that a resonance note 
will be annoying whenever it can be heard, but 
that its audibility is diminished when its dura- 
tion is brief. Therefore, damping a system 
resonance will improve hydrophone quality by 
increasing the broadness of the peak as well as 
by limiting its height. 


4.3.3 Frequency Translation of Airplane 
Noise 

It would be tactically valuable to submariners 
preparing to surface if they could detect the 
presence of hostile aircraft by means of the 
radiated noise and at ranges corresponding to 
at least 30 seconds of flying time. Since this 
type of signal is characterized by a number of 
prominent single-frequency components at the 
very low sonic frequencies (where critical band 
width is less favorable to detection than at 
higher frequencies and where, moreover, the 
sensitivity of the ear and the response of head- 
phones are poor), a study was made^® of the 
possibility that performance could be improved 
by heterodyning the received signal and thereby 



CUDWR-NLL TESTS 


133 


shifting it to a more favorable frequency 
region. 

Recordings were made of the noise received 
from an airplane in level flight, and following 
a straight course, as it approached and then 
receded from a microphone supported just 



FREOUCNCT m CYCLES 


Figure 82. Frequency response characteristic of 
system used in peak tests. Peak had a frequency 
of 1.4 kilocycles and a height of 6 decibels, with 
a Q of 35. 

above the sea surface. Since wind noise, due to 
the flow of air about the receiver structure, 
and the splashing of waves masked the air- 
plane signal at each end of the run, the length 
of time during which the signal-to-noise ratio 
was sufficient to assure signal audibility pro- 
vided a convenient measure of comparative 
performance. The mean intervals of audibility 
were determined for the same group of ob- 
servers listening to a playback of the sounds 
as originally recorded, and then to the output 
obtained by frequency-translating these sounds 
either 200 or 500 cycles. In some of the tests, 
both side bands were presented to the ob- 
servers; in others, one of the bands was elimi- 
nated by means of filters. (When any signal 
component is heterodyned, the resultant sig- 
nal consists of two side bands ; one representing 
the sum of, and the other the difference be- 
tween, the frequencies of the original compo- 
nent and the heterodyne note.) 

No significant quantitative difference was 
found to exist between the intervals of signal 
audibility in the heterodyned and unhetero- 
dyned recordings. However, the listeners found 
the heterodyned sounds qualitatively dissatis- 


fying, apparently because the uniform fre- 
quency shift imposed on each of the signal com- 
ponents destroyed the nearly harmonic relation 
between them which occurs in the sound as 
originally received, and consequently destroyed 
much of the character typifying the original 
signal, thereby increasing its resemblance to the 
random noise background. In other words, 
there seemed to be no actual decrease in ability 
to discern the presence of the wanted sound, 
but less subjective certainty that the sensed cue 
was really the sound wanted. Precautions had 
to be taken during tests with the heterodyned 
sounds to eliminate circuit hum, as well as 
rumbles generated by motion of the tumtable ; 
that is, these sounds were below the audibility 
threshold when the observers listened to the 
unheterodyned recording. 

It would appear to follow that the optimal 
component was not threshold-limited for the 
sounds as originally recorded; on the other 
hand, it is uncertain whether the fidelity of this 
recording, in the very low-frequency region, 
was adequate to the purpose at hand. One 
further possibility is worth considering: that 
a component with a very favorable signal-to- 
noise ratio, inaudible in the unheterodyned re- 
cording because of threshold limitation, was 
rendered audible by frequency translation, but 
that the improvement which might be expected 
from this effect was offset by the accompanying 
destruction of harmonic relations and the help 
they sometimes afford in signal detection (see 
Section 8.5). In further studies along these 
lines, it may be profitable to multiply the com- 
ponent frequencies by a constant factor, there- 
by preserving the harmonic relations instead 
of translating them all by the same amount. 

It may be mentioned, in conclusion, that the 
underwater radiation from ships is generally 
rich in prominent single-frequency components 
in the subsonic range (1 to 10 vibrations per 
second) which are associated with various 
modes of hull vibration. The question whether 
this region could profitably be exploited with 
listening gear designed for long-range detection 
has apparently received no systematic stud 3 \ 


I (restrictedI 


Chapter 5 

FIELD MEASUREMENTS ON MASKING OF TARGET SOUNDS 


A GROUP OF PRACTICAL surveys, made under 
field or semifield conditions, have been de- 
scribed by the New London Laboratory of 
Columbia University Division of War Research 
(CUDWR-NLL) These surveys were under- 
taken in connection with the design, develop- 
ment, and evaluation of practical hydrophone 
installations, and they furnish useful infor- 
mation which is not obtainable from tests of 
the kind already discussed. However, a certain 
amount of caution is required when examining 
the results of field studies since precise acoustic 
measurements are more difficult to make under 
sea conditions than in the laboratory. In addi- 
tion, it is difficult to evaluate, control, and re- 
produce the experimental factors, to vary them 
one at a time, and to assure that the range of 
the variables, known and unknown, which has 
been sampled is either representative or ade- 
quate. In consequence, the quantitative signifi- 
cance of numerical data is often open to ques- 
tion and interpretation. 

The objective of these tests was to study the 
controllable factors which affect the detection 
ranges and bearing accuracies of sonic and 
supersonic installations. However, the methods 
used and the results obtained in these supple- 
mentary studies have considerable intrinsic in- 
terest, and their discussion permits the funda- 
mental problems to be formulated in more con- 
crete terms. The observations on primaudibil- 
ity, that is, on detection range, are summarized 
in the present chapter. The experimental vari- 
ables were hydrophone type, listening band, 
and auditory motion produced by training the 
listening hydrophone or changing the signal 
level. 

5 1 HYDROPHONE DIRECTIVITY IN 
SONIC MASKING 

Before examining the data, it will be use- 
ful to summarize and extend the earlier discus- 
sion of hydrophone directivity given in Section 
3.3 and to correlate some of the properties of 


such devices with those of the ear. Hydro- 
phones discriminate against nonaxial sounds; 
in other words, responsiveness depends upon 
angle of incidence, and the width of the main 
lobe (that angular interval in which response 
is relatively high) is a measure of the discrim- 
ination against nondirectional sounds. This 
lobe width, in radians, is equal to 2\/d, where 
A is the wavelength of the sound and d is the 
linear dimension of the hydrophone, provided 
A is much less than d/2. 

For incident sounds whose half wavelength 
exceeds the dimensions of the hydrophone, 
directivity becomes negligible. Hence, no effec- 
tive discrimination against a nondirectional 
sound field, such as sea noise, can be obtained 
when the half wavelength exceeds the major 
linear dimension of the hydrophone, since there 
will be no significant difference of phase across 
the hydrophone. Thus the condition for direc- 
tivity is A > 2d. On introducing the relation 
\=c/f, where c is the velocity of sound, this 
condition becomes / > c/2d. The velocity of 
sound in sea water depends on temperature, 
salinity, and hydrostatic pressure; a useful 
average value is 4,800 feet per second. 

To be more specific, consider the JP-1 hydro- 
phone, a type frequently used in the New 
London tests. The code designation JP-1 refers 
to a hydrophone fashioned from a hollow nickel 
cylinder about 3 feet long and 2 inches in di- 
ameter. The magnetostrictive nickel tube com- 
pletely encloses a copper coil whose terminals 
are led, through the wall of the tube, to a 
listening amplifier. At the coil terminals appear 
electrical variations induced by the changes in 
magnetic flux set up by incident sound. The 
small diameter of the JP-1 hydrophone pre- 
cludes directivity for underwater sounds inci- 
dent in the plane perpendicular to the geomet- 
rical axis of the cylinder and with frequencies 
below 14.4 kilocycles (c/2d = 4,800/0.33 = 14.4 
kilocycles) . 

In the plane containing the geometrical axis 
of the cylinder, there is useful directivity be- 


^plESTR^ TED 


134 


HYDROPHONE DIRECTIVITY IN SONIC MASKING 


135 


tween 5 and 10 kilocycles. But even in this 
plane, the discrimination of the bare hydro- 
phone is negligibly small (less than 2 
decibels) for frequencies below 1.3 kilocycles 
{c/2d = 4,800/6 - 1.3). With a baffle, which 
shields half of the hydrophone surface, the dis- 
crimination against nondirectional sound is 
approximately 3 decibels greater, at all fre- 
quencies, than is shown by Figure 18 in Chap- 
ter 3 (see also Figure 2 in this chapter) . From 
the preceding discussion, it is clear that the 
JP-1 hydrophone does not have a unique axis. 
Its three-dimensional directivity pattern, when 
the bare hydrophone is immersed in a nondirec- 
tional sound field, has been described as re- 
sembling ‘'a washer with the hydrophone stuck 
through it like a bolt.’' 

The JP-1 hydrophone was standard sonic 
listening gear on submarines and inshore patrol 
craft. The JP-1 is generally mounted on the 
foredeck of submarines, where the conning 
tower serves to shield it from the vessel’s own 
propeller sounds; in addition, the submarine 
may bottom without risking injury to the 
hydrophone. The geometrical axis of the JP-1 
is rotated about its midpoint and in the hori- 
zontal plane. In other words, the plane of the 
“washer” can be trained to different compass 
points; and the absence of directivity in the 
vertical plane is helpful in maintaining contact 
with surface targets at short range. When 
mounted on surface vessels, the JP-1 is gen- 
erally suspended through a well in the hull and 
is then described as throng h-the-hull [TTH] 
gear. The magnetic shielding of this hydro- 
phone is incomplete when it is mounted on sur- 
face vessels. Owing to the high level of ignition 
noise from propulsion machinery and excessive 
stresses in the supporting shaft due to hydraulic 
drag at the higher speeds, TTH gear is usually 
drawn up against the hull and secured with 
its length parallel to the keel when the craft is 
underway. 

It will be noted from Figure 18 in Chapter 3 
that the discrimination of a line hydrophone 
against isotropic sound improves at a rate of 3 
decibels per octave for frequencies greater than 
c/2d. This behavior follows from the fact that 
the width of the main lobe is inversely propor- 
tional to fd; when d is fixed and / is doubled. 


10 log (2fd/fd) = 3 db. (The ordinates are 
negative in Figure 18 of Chapter 3 to show the 
decrease in the level of received sound at higher 
frequencies.) 

The sensitive surface of the type of hydro- 
phone commonly used for supersonic listening 
and echo ranging may be approximated, for 
purposes of this discussion, by a circular disk 
15 inches in diameter (see Figure 17 in Chap- 
ter 3, and Section 3.3). The receiving disk is 
suspended along and rotated about a vertical 
diameter; it is usually protected from the sea 
by enclosure within a sealed liquid-filled metal- 
lic shell or streamlined dome which is mounted 
at keel depth outside the hulls of surface vessels 
and bottomside on submarines. 

While the horizontal and vertical lobe widths 
of a line hydrophone are unequal, leading to the 
“washer” pattern, in the case of a circular disk 
the horizontal and vertical widths are equal. 
The spatial configuration of the main lobe for 
a disk hydrophone is that of a cone with its 
axis perpendicular and its vertex adjacent to 
the hydrophone face. In other words, the disk 
differs from the line in that it discriminates 
against sea noise in the vertical plane as well 
as the horizontal. The discrimination should 
therefore be inversely proportional to (fd)- 
and should improve at a rate of 6 decibels per 
octave at frequencies in excess of 1.9 kilocycles 
{c/2d = 4,800/2.5 = 1.9 kc). These estimates 
are in good agreement with Figure 18 of 
Chapter 3. 

Hydrophone directivity therefore confers 
two advantages: (1) discrimination against 
masking sound which come from directions 
outside the main lobe, although side lobes may 
sometimes be troublesome; and (2) angular 
discrimination, or ability to scan a restricted 
solid angle for a target. This first advantage is 
not directly relevant to masking criteria, 
although it is important in practical perform- 
ance. The amount of this discrimination 
changes with frequency, and therefore alters 
the slope of the masking spectrum, but, within 
wide limits, the slope of a distributed spectrum 
has little influence on primaudibility (see Sec- 
tion 4.2.5) . The second advantage may have a 
considerable effect on the recognition differ- 
ential obtainable in practice. When a hydro- 




RKS'IRICI'KI 


136 


FIELD MEASUREMENTS ON MASKING OF TARGET SOUNDS 


phone with good angular discrimination in the 
search plane is trained across a suspected bear- 
ing at the proper rate, the resultant auditory 
motion should provide a helpful cue in the 
detection of signals, and should also increase 
the operator’s confidence in reporting contacts 
and bearings. 

Consider, for example, the situation in which 
(1) the primaudible target component is a sus- 
tained, unmodulated tone, (2) the background is 
ambient noise, and (3) the mean intensity and 
composition of the received background is es- 
sentially the same at all bearings. If the tone 
radiated by the target is primaudible at the 
listening hydrophone, a change in the quality 
of the received sound occurs when the hydro- 
phone axis crosses the target. This change of 
quality will be detected only when the tone-to- 
noise ratio equals or exceeds the primaudible 
value, but the latter value appears to be several 
decibels lower for a modulated tone than for an 
unmodulated tone (see Section 4.2.2). By re- 
peatedly sweeping the hydrophone axis across 
the target bearing, the operator can, in effect, 
introduce an amplitude modulation in the re- 
ceived signal, or rather, in the signal-back- 
ground mixture, and thereby detect a fainter 
tone than would otherwise be possible. 

The locally produced time pattern should be 
most effective (1) when the level of the mix- 
ture varies over a range of 1 decibel or more, 
since smaller changes are likely to escape detec- 
tion, and (2) when the change is repeated at a 
rate of about 3 cycles, since slower changes 
place a greater burden on the memory, and 
faster ones become difficult to perceive. For 
optimal results, therefore, the intensity of the 
received mixture should pass smoothly from its 
lower to its upper limit in about 0.16 second, 
and perform the reverse change in a like time ; 
in other words, the level of the mixture should 
rise or fall at a rate of about 7 decibels per 
second (1 decibel per 0.16 second). If practica- 
ble, the transit should be repeated several times 
at a regular rate of about 3 sweeps per second ; 
therefore the angular limits of train should not 
greatly exceed those needed to drop the non- 
axial intensity by more than the required 1 
decibel. 

Let I represent the average intensity of the re- 
ceived background, in the critical band centered 


at the frequency of the primaudible tonal com- 
ponent in the signal; % the peak intensity of 
the tone, received in the on-target position ; and 
i some lower intensity of the tone, received 
when the hydrophone is trained </> degrees off 
target. From the preceding discussion, the 
hydrophone must be trained between limits 
such that (I + ip) / {I +i) = 1.2b, since 10 log 
1.25 = 1 db. This expression contains three un- 
knowns ; therefore one additional condition 
must be imposed before it is possible to obtain 
a relation between i and ip, and thereby deter- 
mine (f>, the number of degrees the hydrophone 
must be trained off target during the process of 
sweeping. This additional condition is the value 
of ip/I, the ratio at primaudibility between the 
recurrent peak intensity of a modulated tone 
and the intensity of the distributed background 
included within the critical band centered at 
the tone frequency. This value cannot be stated 
with certainty. From Table 4, in Chapter 4, a 
reasonable value appears to be ip/I=V 2 - For 
the sake of generality, however, two additional 
possibilities are considered: ip/ 1=1 and 
In decibels, these correspond, respec- 
tively, to a tone primaudible when its level is 
0, 3, and 6 decibels below the level of back- 
ground within the critical band. The values of 
i/ip obtained from the first condition, upon sub- 
stituting each of the three alternative values of 
the second, are listed in Table 1 in arithmetic 
and also in logarithmic form. 

It is clear from the last column of Table 1 
that even 100 per cent modulation is not likely 
to render the tone primaudible when its recur- 
rent peak level is about 6 decibels below back- 
ground within the critical band which includes 
the tone. Furthermore, improvement as great 
as this may be expected only if it is valid to as- 
sume that an increase of 1 decibel in a single 
critical band can be perceived, even though no 
corresponding changes occur in the other 
critical bands stimulated by background. 
Table 1 also implies that when sweeping a 
hydrophone across the target, the sweep should 
be great enough to reduce the target sounds by 
at least 3 decibels from the level received on the 
hydrophone axis (see bottom line of table). 

Thus, for many practical purposes, it seems 
most suitable to adopt the angle between the 


RKSTRICTED I 


HYDROPHONE DIRECTIVITY IN SONIC MASKING 


137 


axis and the —3-decibel point as numerically 
defining the angular discrimination of a hydro- 
phone. This is generally about one-fourth the 
width of the main lobe, defined in Section 3.3 
as the angle between the two directions of mini- 


Table 1. Estimated effect of modulation on 
detection of a masked tone. 


/ 

] 0 log -- in db 

0 

0.5 

-3 

0.25 

-6 

i 

0.6 

0.4 

0.0 

i-p 

i 

10 log —in db 

-2 

-4 

00 

H 





mum response on each side of the hydrophone 
axis. 

When marked modulation, or a similar time 
pattern in the incident signal, does not make 
the process superfiuous or harmful, it would 
seem that sweeping a directional hydrophone 
through the target bearing at the optimal rate 
should improve the detectability of tones by 
2 to 3 decibels. The value of this listening tech- 
nique in practice may be better appraised if it 
is recalled that when signals are slowly faded 
into the presented sound, as in the 2-hour tests 
described in Section 4.2.6, the performance of 
observers becomes inconsistent and generally 
requires 2 to 3 decibels more gain in the signal 
channel to assure detection. It seems unlikely, 
however, that the combined improvement of 4 
to 6 decibels above performance in the 2-hour 
tests would usually be obtained in the field be- 
cause the noise background, in contrast to what 
was assumed in the preceding discussion, may 
itself show a disconcerting dependence on 
hydrophone orientation, and because, as in- 
dicated later, there is probably no unique op- 
timal rate of train and no practicable method 
of maintaining such a rate if it could be 
specified. It seems likely, however, that the 
field RD may be taken as essentially equal to 
the random-order RD when the operator can 
train a directional hydrophone. It should be 
noted here that the on-off effect, in addition to 
improving responsiveness to a given signal 


component, may offset the effects of careless 
search or poor presentation, and thereby in- 
crease the chance that an operator will detect 
the optimal component (see Section 4.2.6). 
Familiar illustrations, from the field of vision, 
of the principle that intermittent stimuli may 
be more attention compelling than sustained 
stimuli of the same kind are the flashes re- 
ceived from lighthouse beacons and from illu- 
minated advertising displays. A few of the 
tests described in Section 5.6 indicate that the 
ability to detect the optimal component is en- 
hanced by sweep modulation. 

To apply the preceding discussion, consider a 
JP-1 hydrophone; let the signal be a sustained 
5-kilocycles tone, radiated by a distant target, 
and just audible in the presence of isotropic 
noise. The width of the main lobe is equal to 
2k/d, which at 5 kilocycles gives 0.64 radian, 
or 37 degrees (compare with Figure 16 in 
Chapter 3) . The angular discrimination (or ap- 
proximate arc which must be described to pro- 
duce a detectable change in the character of the 
mixture received in the on-target position) is 
one-quarter of the lobe width, or 9 degrees. 
Thus the total angular sweep is 18 degrees. 
For an assumed modulation rate of 3 per 
second, this corresponds to an average sweep 
rate of 54 degrees per second, or about 8 rpm 
(see Table 2). Under the same conditions, a 5- 
foot line would have to be trained at only % 
the rate of the JP, because the lobe width is 
inversely proportional to hydrophone lengths 
The optimal rate of sweep also depends on the 
frequency of the primaudible signal, since lobe 
width is inversely proportional to frequency. 
From these relations, estimates may be made 
of the optimal rate of sweep, and also of the 
angular discrimination and bearing accuracy, 
defined as the rms bearing error. A group of 
such estimates, showing the dependence upon 
hydrophone length and listening frequency, are 
given in Table 2 and are in good agreement 
with observation. The values for a 1-foot hydro- 
phone at a frequency of 1 kilocycle are proba- 
bly overestimates, since even an inefficient 
baffle will produce a noticeable change in char- 
acter for off-axis orientations exceeding 90 
degrees; these values have therefore been set 
off in parenthesis. 





138 


FIELD MEASUREMENTS ON MASKING OF TARGET SOUNDS 


It will be observed that all the quantities 
tabulated decrease with increasing frequency 
and increasing length. The assumptions made 
in computing bearing accuracy are discussed 
later in this section. 


rate and resulting angular acceleration for best 
aural performance is inversely proportional to 
the first power of the length. Other mechanical 
problems also become serious for long line 
hydrophones. For example, relatively small 


Table 2. Estimated optimal sweep rate, angular discrimination, and bearing accuracies of line hydro- 
phones for tonal signals detected in the presence of isotropic noise. Values in parentheses are probably 
overestimates if the hydrophone is supplied with a baffle. 


Frequency of tonal signal in kc 


Hydrophone 

length 

in 

feet 


1 



5 



10 


Optimal 

Angular 

Bearing 

Optimal 

Angular 

Bearing 

Optimal 

Angular 

Bearing 


rate 

discrimina- 

accuracy 

rate 

discrimina- 

accuracy 

rate 

discrimina- 

accuracy 


in 

tion in 

in 

in 

tion in 

in 

in 

tion in 

in 


rpm 

degrees 

degrees 

rpm 

degrees 

degrees 

rpm 

degrees 

degrees 

1 

(120) 

(±135) 

(±45) 

24 

±27 

±9 

12 

±14 

±5 

3 

40 

± 45 

±15 

8 

± 9 

±3 

4 

± 5 

±2 

5 

24 

± 25 

± 8 

4.8 

± 5 

±2 

2.4 

± 3 

±1 

8 

15 

± 15 

± 5 

3 

± 3 

±1 

1.5 

± 2 

±1 


The total angle of sweep under optimum 
conditions is twice the angular discrimination, 
given in the second column under each fre- 
quency. In practice, mechanical difficulties of 
training the hydrophone may require slower 
rates of sweep than those shown, or, alterna- 
tively, a wider sweep, which also would lead to 
an effective reduction in the rate of modulation. 
In particular, for hand-trained JP gear the 
moment of inertia of the hydrophone would 
make it difficult to maintain an angular oscil- 
lation at the indicated rate over the required 
angular limits, that is, 3 sweeps per second. 
Figure 64 in Chapter 4 suggests, however, that 
the rate of modulation can be changed by a 
factor of 2 without changing the recognition 
differential by more than 1 decibel. 

It may be inferred that short, mechanically 
trained lines are undesirable because of their 
broad response patterns and poor discrimina- 
tion against isotropic noise, and because of the 
high rates of train usually needed. Extremely 
long, mechanically trained lines would give 
good noise discrimination and good angular dis- 
crimination, but would be mechanically cumber- 
some; large forces would be required to train 
them, since the moment of inertia increases as 
the cube of the length, while the angular sweep 


amounts of pitch and roll may make it difficult 
to maintain contact with a target, or to scan a 
given bearing effectively, when the width of the 
main lobe is too narrow. 

There is, however, no reason to believe that 
any single length is optimal, or that any par- 
ticular ratio of train is best. Practical listening 
situations can vary over a large range, and 
primaudible signal frequencies will vary cor- 
respondingly. A few general and fairly rough 
rules can be suggested from estimates^^ of the 
probable frequency at which primaudibility 
will occur under different operating conditions, 
but the forces and rates of train which would 
be required evidently cannot be maintained 
during an entire watch while searching for con- 
tacts. There is no justification, under the cir- 
cumstances, for optimistic views concerning 
the inherent superiority of recognition differ- 
entials which can be obtained in the field with 
the angular scanning technique, as against re- 
cognition differentials obtained in the labora- 
tory by the random-order method. But there is 
little doubt that performance in the field will 
be improved by using a listening hydrophone 
with good angular discrimination and that less 
uncertainty will be experienced at first contact 
when selective, angular scanning is possible. 


HYDROPHONE DIRECTIVITY IN SONIC MASKING 


139 


The estimated bearing errors listed in Table 
2 have been computed by assuming (1) that 
errors (made in determining the midpoint be- 
tween the angular limits of train) follow an 
approximately normal distribution; (2) that 
the mean value of the distribution, in the 
absence of systematic error, is indistinguish- 
able from the true bearing; (3) that the num- 
ber of reported sonar bearings which deviate 
from the true bearing by an amount exceeding 
the average angular limits of train is negligible ; 
and (4) that the change in true bearing during 
the several successive transits required to ob- 
tain a sonar bearing may be ignored. It is a 
commonly used rule-of-thumb that practically 
all of a normally distributed population is in- 
cluded between limits which differ from the 
mean by So-, where o- is the standard or rms 
deviation. Accordingly, if the angular limit of 
train be equated to 3o-, the estimated standard 
deviation of the bearing errors is one-third of 
the average limit of train; this criterion was 
used in obtaining the bearing errors listed in 
the table. The rms deviation from the mean in 
a normal distribution includes or more 
exactly 68.3 per cent, of all the cases; in other 
words, if the preceding analysis is valid, two of 
every three bearing determinations will be in 
error, on the average, by no more than the 
tabulated values. While this argument is not 
precise, it yields conclusions in good agreement 
with rms bearing errors measured for a variety 
of conditions.®'® 

Because of selective attenuation, the sonic 
detection of targets which become primaudible 
at ranges in excess of 7,000 to 10,000 yards 
tends to occur at frequencies below 2 kilo- 
cycles.^2 In this region, the required rates of 
train are high and the available bearing accu- 
racies relatively poor (see Table 2). This con- 
clusion agrees with the practical observation 
that the bearing accuracy of 3-foot lines de- 
teriorates when the target vessel is detected at 
very long range, according to an informal com- 
munication from CUDWR-NLL. At 1 kilo- 
cycle, the discrimination of the JP hydrophone 
against deep-sea ambient is about 2 decibels, 
that of a 5-foot line is about 4 decibels, and 
that of an 8-foot line is about 6 decibels (see 
Figure 18 in Chapter 3, and the preceding dis- 


cussion) . These same relative differences apply 
to all frequencies between 0.5 and 10 kilo- 
cycles. It is possible that similar statements 
apply to self-noise, but there is no direct evi- 
dence on this point. The different amounts of 
discrimination against nondirectional back- 
ground imply that detection ranges will be 
somewhat greater for the longer lines and that 
the primaudible frequency, i.e., signal-to-noise 
ratio in optimal band, may vary with the listen- 
ing hydrophone for a given signal (see Table 3 
in Chapter 4). When the frequency at which a 
given target becomes primaudible does depend 
on hydrophone length, the quantities listed in 
Table 2 should be compared along a diagonal 
rather than within a column. 

When the signal-to-noise ratio is equally good 
over a fairly wide band of frequencies, training 
the hydrophone will change the received level 
of the higher frequencies more rapidly than 
that of the lower, because the width of the 
main lobe decreases with increasing frequency. 
If the signal-to-noise ratio is well above the 
value required for primaudibility, a change in 
quality will be heard as the hydrophone is 
trained through the target. However, at prim- 
audibility it is likely that the better time pat- 
tern in the high-frequency portion of the 
optimal band will render it more audible than 
the low-frequency portion, and the effective 
band of primaudible frequencies will be nar- 
rower than might be expected on the basis of 
the signal-to-noise ratio alone. 

Since most target spectra are distributed 
sounds, the changes in quality just mentioned 
may assist in obtaining sonar bearings when 
the signal is well above primaudibility. To take 
advantage of this factor, however, the operator 
must be able to concentrate on the high signal 
frequencies and disregard the low ones, since 
the target will be audible over a fairly wide 
band of frequencies and the intensities of the 
low-frequency components will change rela- 
tively slowly with angle of train. If the opera- 
tor is unable to ignore the low-frequency com- 
ponents, bearing accuracy will suffer. This 
problem may be met by inserting the proper 
high-pass filters, if these are available, or by 
dropping the gain until the signal approaches 
the audibility threshold. The effect of the latter 



STRTCTE 


f 


140 


FIELD MEASUREMENTS ON MASKING OF TARGET SOUNDS 


is equivalent to introducing a high-pass filter 
(see Figure 5 in Chapter 4). 

The discussion of the present section is in- 
tended to serve as a basis for coordinating and 
assessing observations described in the re- 
mainder of this chapter. 


52 TECHNIQUES 


and at all frequencies between 0.1 and 10 kilo- 
cycles. The projector and listening hydrophone 
were 40 feet apart. Acoustic coupling effects 
between a broadly tuned source and receiver 
are probably negligible at this distance,^ but 
the curvature of the wave front incident on 
a hydrophone so close to the source may pro- 
duce significant variations in receiver response. 
The two circular arcs in Figure 1 represent 


The listening tests®'®’^® were conducted aboard 
a laboratory vessel anchored offshore. A signal 
of controlled intensity was radiated by an 
underwater projector mounted astern, and the 
hydrophone under test, suspended to the depth 
of the underwater projector through a well in 
the hull, picked up a mixture of signal and 
background. The latter consisted largely of 
water noise and sounds produced by the slap- 
ping of waves against the hull. Thus, the 
character of the mixture received at various 
orientations depended upon the properties of 
the hydrophone in use. Since the distance be- 
tween the listening hydrophone and the arti- 
ficial target was fixed, and the energy radi- 
ated by the target could be controlled, it was 
possible to evaluate recognition differentials 
under semifield conditions. However, variabil- 
ity of such factors as ambient noise level and 
transmission conditions, such as are usually 
found in the field, may have influenced the 
results. 

The four signals used were originally ob- 
tained as high-quality disk recordings. The 
recording hydrophone was nondirectional and 
had uniform response over the sonic or re- 
cording band. The sounds radiated by the 
four target vessels were received in quiet water 
and at a range of about 200 yards. For the 
listening tests, a suitable portion of a disk 
recording was impressed on an endless mag- 
netic tape capable of continuously repeating 
a signal 20 seconds long. The magnetic play- 
back technique has the advantage of extend- 
ing the useful life of the disk, and is reliable 
even when the listening-test vessel sways with 
large amplitude. 

As used in these tests, the underwater pro- 
jector may be considered a point source of 
sound, radiating equally well in all directions 



Figure 1. Spherical wave front incident on a 
hydrophone. In the plane of the figure, the re- 
ceiver has a length d. The radial distance from 
the source s to the midpoint of the receiver is r, 
and to either end of the receiver is r h. Hence, 
(r-\-h)~=r--\-d-/4; or, provided the term in )i- may 
be neglected, h — d‘/8r. If /i = X/4, f = 2rcld\ 
where X is the wavelength, c the velocity, and / 
the frequency of the incident sound. For r = 40 
feet, d = 8 feet, and c = 4,800 feet/second, f = 
2rc/d- = 6 kilocycles and X << 4r (as assumed in 
dropping the term involving h ') . 

parts of a progressive spherical wave which 
are separated by a distance h equal to one- 
quarter the wavelength. In this condition, the 
ends of the receiver lag its center by 90 de- 

a Absence of electrical interaction between the signal 
and listening hydrophone circuits was demonstrated by 
surrounding the projector with several layers of Air- 
foam rubber, to reduce radiation into the water, and 
then driving the source at maximum electrical power. 
Under these conditions, no measurable or audible sig- 
nals were observed at the outputs of the test hydro- 
phones. 


SIGNALS AND BACKGROUNDS 


141 


grees. For larger angles of lag the integrated 
output from the various segments of the hydro- 
phone surface begins to drop sharply, due to 
mutual phase cancellations. It follows that, 
for frequencies exceeding 2rc/dr, the axial hy- 
drophone response falls below the value ob- 
tained with a plane wave. For an 8-foot line- 
hydrophone, the longest used in these tests, 
the limiting frequency is 6 kilocycles; for a 
4-foot line, good response is maintained up to 
24 kilocycles. When curvature of the incident 
wave front impairs response, it also broadens 
the effective lobe width, since phase differences 
among segments of the receiving surface are 
changed less in orienting from on-axis to off- 
axis positions. It is evident that these effects 
resulting from curvature of the wave front 
were probably negligible in these tests, except 
possibly with the 8-foot line. 

Measurements of the received signal, or lis- 
tening hydrophone output, were made only 
when the axis of the test hydrophone was on 
target and the overall level of the received 
signal was well above overall background. The 
background present was also measured while 
the projector was silent. These measurements 
of received levels were made with a meter 
adjusted to follow peak amplitudes. For most 
sounds measured, the indications given by this 
meter fluctuated from the mean reading, which 
was used in computing recognition differentials, 
by about ±2 decibels. The overall readings 
measured power in the 0.2- to 10-kilocycle band. 

No accurate and independent measurement 
of the signal level received at primaudibility 
can be made under field conditions, because 
that part of the listening hydrophone output 
corresponding to signal is a small fraction of 
the total due to signal and background, and 
because fluctuations in the level of the listen- 
ing hydrophone output usually exceed the con- 
tribution made by the signal. In order to evalu- 
ate field recognition differentials it is conse- 
quently necessary to know the relation between 
the level of the j:’eceived signal and the known 
input to the projector. The recognition differ- 
entials given later in Figures 5 through 7 
were obtained by assuming that the intensity 
of the received signal was directly proportional 
to the power dissipated in the projector circuit. 


in other words, by assuming that measurement 
of the power dissipated, at and above primaudi- 
bility, and of the power received, above primau- 
dibility, permit calculation of the received 
primaudible signal power by simple proportion. 


5 3 SIGNALS AND BACKGROUNDS 

Four signals were used, and these showed 
various degrees of amplitude modulation. Three 
of the signals were recordings of submarine 
sounds ; the fourth, a recording of sounds from 
a surface vessel. The character of the subma- 
rine sounds, at and above primaudibility, is 
described in the legend of Figure 5. It should 
be noted that these descriptions apply to a 
specific set of conditions — hydrophone, listen- 
ing band, signal level, among others. The char- 
acter of the ship signal, above primaudibility, 
is described in Section 5.6. 

Frequency analyses were obtained for the 
three submarine signals. These were made by 
scanning the output of the test hydrophone, 
in the interval 0.2 to 10 kilocycles, with a 50- 
cycle band-pass filter of variable midfrequency. 
During such an analysis, the hydrophone axis 
was held on target, the received overall signal 
level was well above overall background, and 
the power dissipation in the projector circuit 
was maintained at a measured value. The lev- 
els in successive 50-cycle bands, relative to the 
overall level of the received sound, were ob- 
tained with a power level recorder adjusted 
to read rms amplitudes. When the sounds mea- 
sured are of constant amplitude, the joint use 
of a peak indicator for overall measurements 
and an rms indicator for analysis involves no 
difficulties. But when the sounds fluctuate and 
the resolution time of the indicator affects the 
reading, the relation derived between signal 
and background at primaudibility may be dif- 
ferent from that found by the methods of Sec- 
tion 4.2.2. Levels in the various 50-cycle bands 
were found to fluctuate by between zb 3 and 
zb 5 decibels from the mean levels which were 
used to plot the spectra. The magnitude of 
this variability and the observation that the 
power level recorder shows more fluctuation in 
a 50-cycle band than in a 10-kilocycle band 


ESTRTCTED 


3 


142 


FIELD MEASUREMENTS ON MASKING OF TARGET SOUNDS 


agree with the data described in Sections 4.2.2 
and 4.2.3. 

The spectra in Figures 5 through 7 were 
transcribed from the power level recorder trace 
by reading the relative levels at 100-cycle in- 
tervals below 1 kilocycle, and at either 500- 
cycle or 1,000-cycle intervals above 1 kilocycle ; 
levels at intermediate frequencies were plotted 
only when they deviated markedly from the 
general trend. Successive points in the plotted 
spectra were connected by straight lines. This 
method of representing the 50-cycle analysis 
lacks the clarity and precision of the point-by- 
point procedure used in preparing the spec- 
tra shown in Section 4.2.2, but when the de- 
picted sound resembles cavitation or ambient 
noise, the two methods should yield essentially 
the same trend. 

The spectra in Figures 5 through 7 repre- 
sent the hydrophone output and do not exhibit 
the characteristic negative slope shown by the 
spectra illustrated in Sections 4.1 and 4.2. The 
latter were obtained with recording hydro- 
phones which had an approximately uniform 
response over the recording band. The pro- 
jected signals used in the New London tests 
also had the typical negative slope of ship 
sounds, but receiver response increased with 
frequency and thus offset the falling character- 
istic of the sound-in-the-water. Figure 2, for 



I 10 

FREQUENCY IN KC 

Figure 2. Frequency response of the JP-1 
hydrophone-baffle assembly in a plane-wave field. 


example, shows the frequency response of a 
JP hydrophone. The ordinate gives response 
in terms of output voltage generated by a plane 
wave, incident along the acoustic axis, which 
has an rms pressure of 1 dyne per square 
centimeter at each calibrating frequency. The 

f ^Ksri 


indicated response characteristic rises at a 
rate of about 9 decibels per octave and drops 
abruptly at frequencies above 20 kilocycles. 

This behavior is due to the combined effect 
of several factors, of which two will be men- 
tioned here. To begin with, the voltage gen- 
erated by variations of magnetic flux is pro- 
portional to dp/dt, the time rate of change of 
the incident pressure, and dp/dt, in turn, is 
proportional to the incident frequency. This 
factor alone would cause output to increase 
at a rate of 6 decibels per octave, since the 
power is proportional to the square of the 
generated voltage and hence to the square of 
the incident frequency. The drop above 20 
kilocycles is due to the fact that the 2-inch 
diameter of the magnetostrictive cylinder is 
less than sonic wavelengths; thus, diffraction 
occurs and there is no well-defined acoustic 
shadow at the rear of the cylinder. Near 20 
kilocycles the pressure variations, at the por- 
tions of the cylinder surface facing toward 
and away from the source, are 180 degrees out 
of phase; in other words, the diameter of the 
cylinder equals a half wavelength. Therefore 
the integrated response falls. 

Practical listening installations whose re- 
sponse increases with frequency tend to com- 
pensate for the negative slope of ship signals 
and to present a flat spectrum to the head- 
phones, as shown in Figures 5 through 7. Thus 
the relative loudness contribution from the 
middle frequencies is raised, and it becomes 
difficult to increase the sensation levels at the 
low and high frequencies without producing 
discomfort (see Section 4.2.5) . 

The masking background used in these tests 
was thoroughly realistic but varied from time 
to time in composition and intensity. In order 
to obtain reliable comparisons between hydro- 
phones or observers under these circumstances, 
it is necessary either to monitor the back- 
ground constantly, to conduct a group of si- 
multaneous tests with one target and several 
hydrophones, or to run tests on different hydro- 
phones in rapid succession. Thus, field studies 
are inherently more complex than laboratory 
tests and may yield less precise results. 

The major sources of background were white- 
caps, waves breaking against the hull, distant 


SIGNALS AND BACKGROUNDS 


143 


surf and harbor traffic, activities on board (in- 
cluding training of the test hydrophone), and 
system noise. Contributions from water mo- 
tion depend on wind and sea states and on the 
orientation at which the listening vessel re- 
ceives the impact of waves. Turbulence about 
a rapidly trained hydrophone, as well as me- 
chanical vibrations associated with the process 
of training, undoubtedly contribute to the re- 
ceived background. This factor was not taken 
into account in measuring background levels, 
probably because such effects were too small 
to attract attention and hence too small to af- 
fect the results. 

Figures 5 through 7 indicate that the com- 
position as well as the intensity of received 
background changed with sea state, and that 
noise discrimination is better with long hydro- 
phones and at high frequencies. The measured 
level of overall background received by a given 
hydrophone was 8 to 10 decibels higher for sea 
state 3 than for sea state 1 ; this increased 
contribution was restricted to the low and mid- 
dle frequencies, with an actual decrease oc- 
curring in the high-frequency region. The re- 
port * describing these tests discusses the change 
of background with sea state as follows : “This 
type of change in the background spectrum be- 
tween sea states 1 and 3 was caused chiefly 
by the great increase in the size and number 
of waves slapping the hull of the listening 
vessel, thus giving a rise in low and middle 
frequency levels ; but at the same time, the num- 


ber of whitecaps in the sea state 3 was not 
quite as great as the number present in the sea 
state 1 because of differing wind conditions, 
and as a result high frequency levels were lower 
for sea state 3.” This type of background be- 
havior is somewhat unusual since these par- 
ticular tests were conducted in a cove. The 
shoreline shielded the water from the full 
impact of the wind in the state 3 tests, but 
not in the state 1. Hence, the wave height, but 
not the number of whitecaps, corresponded to 
a number 3 sea in open water. It is possible, 
however, that the effect is general and that with 
increasing sea state, the effect of wave slap on 
the hull may increase the received noise back- 
ground at low frequencies more than wave slap, 
whitecaps, and other factors increase the high- 
frequency noise components (see also Figure 
4 in Chapter 4). 

Figure 3 shows nearly simultaneous power 
lever recorder traces of background received in 
the 0.2- to 10-kilocycle band by several hydro- 
phones with different directivity indices. Refer- 
ence 8 discusses these traces as follows : “In sea 
state 3 there were many localized sources of 
noise arising from waves breaking near the 
listening vessel and wave slap on the hull. The 
random occurrence of these noise sources gave 
integrated ‘smooth’ character to the sound re- 
ceived in the nondirectional hydrophones. How- 
ever, when hydrophone directivity was intro- 
duced the number of sources affecting the char- 
acter of sound was reduced and the integration 


30 


u 25 


> 5 

iij < 
-I (T 


20 


10 


NONDIRECTIONAL 
-HYDROPHONE — 




h 


10 SECONDS 




Figure 3. Time-amplitude pattern of water noise received by directional and nondirectional hydrophones. 
Mean levels of the traces are arbitrary and are not responsible for differences in the relative amplitudes of 
noise peaks. 


STRICTE 





144 


FIELD MEASUREMENTS ON MASKING OF TARGET SOUNDS 


of the sounds was not as complete ; so that the 
individual sources began to become more distin- 
guishable as individual splashes. Hence, the 
greater the directivity became the more im- 
portant was each source of noise near the main 
lobe of the hydrophone. As a result the ampli- 
tude-time pattern of the directional hydro- 
phones had much greater peaks and valleys 
than the nondirectional hydrophones. This is 
illustrated in Figure [3]. During the tests, 
comments of the listeners indicated that when 
listening with directional hydrophones their 
attention was focused only on the valleys of 
the amplitude-time pattern of the background 
noise and the peaks were ignored. Hence, the 
effective masking level of the noise was less 
than the average level; and since the volume 
indicator used in the tests responded to values 
between peak and rms readings, the calculated 
values of RD would be greater for patterns 
such as those in Figure [3C] than in Figure 
[3A] The effect of “plonks” in received water 
noise is also illustrated in Figure 4 of Chapter 
4; in that case also the effect of wave slap 
raised the low-frequency and middle-frequency 
levels to a greater extent than those at the high 
frequencies. It will also be noted that the simu- 
lation of hydrophone directivity described in 
Section 4.1.8 made no allowance for changes 
in the peakiness of background received with 
directional hydrophone. 

The horizontal lines superposed on the traces 
in Figure 3 indicate how levels of masking 
background may be read from such traces in 
order to compute recognition differentials and 
to obtain the relation between signal and back- 
ground spectra at primaudibility. It is prob- 
ably justifiable to disregard the possibility of 
persistent masking (see Section 9.2.2), when 
the successive noise peaks occur several sec- 
onds apart, as in the case illustrated; thus, it 
is doubtful whether limiters would be of much 
help under these conditions. It might be use- 
ful in further recognition studies (in which 
the masking background fluctuates widely and 
so slowly that resolution time of the indicator 
does not falsify results) to examine the time 
pattern in the primaudible, as well as in the 
overall band. It is also worth inquiring whether 
or not increased peakiness in the background 


received by directional hydrophones increases 
the difficulty of maintaining faint contacts and 
getting accurate sonar bearings. 


5 4 TEST METHODS 

About a dozen different hydrophones were 
studied in this test program. They were usu- 
ally supplied with a baffle and were coupled to 
the listening amplifier through one of a set of 
impedance-matching transformers. The per- 
formance of hydrophones is determined by a 
complex set of factors. In addition to those 
already discussed, such as angular and noise 
discrimination, there should be mentioned fre- 
quency response, listening band width, impe- 
dance match to amplifier, distortion, absolute 
response, and system noise. If the installation 
is unsatisfactory in any of these respects, sig- 
nal detection may deteriorate. Little detailed 
information about these hydrophones, along 
the lines just specified, is currently available; 
hence, observations on comparative perform- 
ance must be treated with caution in so far as 
the general problem of detection is concerned. 
It should be pointed out, however, that the 
major function of these comparative hydro- 
phone tests was to select from among avail- 
able designs the one best adapted to serve a 
specific purpose. 

The nominal listening band used in this 
study extended from 0.2 to 10 kilocycles, but 
in a number of the tests high-pass filters were 
inserted in the amplifier circuit. The approxi- 
mate values of the low-frequency cutoffs, as 
well as the effect of the filters in modifying 
the spectra of received signal and background, 
are shown in Figure 6. The characteristics, 
including insertion loss, of a similar filter set, 
built into the JP-1 amplifier unit, are given in 
Figure 4. The “bass-boost filter” indicated in 
Figure 4 is essentially a broadly tuned reso- 
nance network capable of accentuating the low- 
frequency range without unduly attenuating 
the high. 

The effective listening band width is also 
determined by the properties of the headset. 
Two types of headsets, a crystal and a mag- 
netic, were used during the course of these 



TEST METHODS 


145 


tests. While both types of phones were of 
high quality, their response characteristics'^ 
dropped sharply above 7 kilocycles. 

Amplifier gain was set for a comfortable 
listening level. Seven different presentation 



Figure 4. Response of the JP-1 amplifier for 
various high-pass filter positions. Numerals 
designate nominal low-frequency cutoff in cycles. 

methods were tried at various times, but only 
one of these was used in a given test. The 
purpose of varying presentation method was 
to evaluate the effects upon signal detection 
of auditory motion introduced by training a 
hydrophone and by opening or closing the 
range between the target and listening vessel. 
The two extremes of auditory motion among 
the seven methods examined correspond to the 
sustained and the interrupted signals used in 
the laboratory tests described in Section 4.2.6 ; 
in other words, the hydrophone axis was fixed 
on the projector bearing, the gain in the pro- 
jector circuit was maintained at a selected level, 
and a disabling switch made it possible to radi- 
ate either a sustained signal or one which was 
completely blanked for brief intervals. Clearly, 
the use of a blanking switch makes it possible 
to get much sharper off -on effects than can be 
approached in practical listening unless ex- 
tremely directional hydrophones are used. 
While care is necessary to eliminate false cues 
due to key clicks and switching transients, the 
chief effect of these disturbances is usually to 
scatter signal energy beyond the limits of the 
frequency band containing the uninterrupted 
signal (see Section 7.1) ; hence, transients are 
not likely to falsify results when the masking 
background extends over the entire sonic lis- 
tening band. 


To test the effects of increase or decrease in 
target range when the listening hydrophone is 
not trained or has poor angular discrimination, 
the axis of the test hydrophone was fixed on 
target and the level of the radiated signal 
raised or dropped at the rate of 0.2 decibel 
per second. This is a rather low rate of audi- 
tory motion but probably defines the upper 
limit of rates likely to be encountered in prac- 
tical listening with a fixed or nondirectional 
hydrophone. Even at a range as short as 1,000 
yards and a speed of about 10 yards per second 
(18 knots), the change of average intensity 
amounts to only 0.09 decibel per second, pro- 
vided the intensity is assumed to fall off as the 
square of the distance. In the 50 seconds re- 
quired to close the range from 1,000 yards to 
500 yards, on these assumptions, the sound level 
will increase by only 6 decibels, giving an av- 
erage change of 0.12 decibel per second during 
this interval. At longer ranges or with slower 
ship speed, even smaller rates of change will 
be found. It should be noted, however, that 
much larger intensity rates may be produced 
occasionally by factors causing rapid fluctua- 
tions in transmission (see Figure 13 in Chap- 
ter 3), but such fluctuations are irregular and 
may cause the signal-to-noise ratio to fall 
below the primaudible value and thus hamper 
detection. 

Three additional presentation methods were 
devised in order to study the effect of joint 
motion of the hydrophone and the target. In 
each case, the test hydrophone was trained 
through the projector bearing at about the 
optimal speed, and the level of the radiated 
signal was held at a selected value or changed 
progressively at a rate of 0.2 decibel per sec- 
ond. It is possible that such effects could be 
simulated adequately in the laboratory by using 
the proper combination of filter elements and 
time constants in the signal channel, and syn- 
chronizing their action with the motion of a 
bearing wheel controlled by the subject. 

To determine primaudible signal levels for 
the various presentation methods, the gain set- 
ting in the projector circuit was changed and 
a tabulation made of power level and correlated 
listener response. Primaudible components 



146 


FIELD MEASUREMENTS ON MASKING OF TARGET SOUNDS 


were identified solely on the basis of the ob- 
servers' subjective impressions. 

When several listeners were available simul- 
taneously, a test consisted of a group of 20 to 
30 presentations, each lasting about 10 seconds, 
and the test administrator varied successive 
signal levels in random fashion. When the 
sound level was constant during presentation, 
the observers removed their phones between 
presentations, in other words, they did not 
listen while the signal was being adjusted to 
its new level. When signal level changed con- 
tinuously during the listening interval, each 
observer noted the time at which the signal 
became audible or inaudible. From the re- 
corded times, the mean result for the group 
could be determined. During group tests, the 
hydrophone was controlled by the test admin- 
istrator, who judged the proper rate and limits 
of train by aural monitoring. In practical lis- 
tening, the coordinated activity of an operator's 
arm and ear may assist signal detection; con- 
versely, the observers in these tests had the 
advantage of knowing that a signal was usu- 
ally present in the received mixture, although 
occasional checks were run with the projector 
disabled. 

A large fraction of the recognition differen- 
tials were obtained in self-administered tests 
performed by experienced listeners, who 
started with an inaudible signal level and in- 
creased it in steps of 2 decibels until the masked 
threshold was reached. In some cases, the rate 
and direction of change was determined by the 
presentation method selected. Self -testing was 
preferred for its brevity and also made it pos- 
sible for the listener to train the hydrophone 
himself. 

The inconstancy of background made it diffi- 
cult to correlate the various methods of test 
administration and scoring. A few comparison 
checks were made, however, and the observed 
variation among the averages obtained for any 
two techniques or groups of listeners did not 
appear to exceed 1 to 2 decibels. Mean results, 
based on groups of between 2 and 8 observers, 
were obtained under as nearly constant back- 
ground conditions as possible, and the index 
sought was that signal-to-noise ratio at which 
only half the group could hear the signal. No 


transition curves are available for these tests. 
However, in Chapter 4, Table 6, and Figures 6 
through 23 imply that the slopes of such curves 
are not strongly dependent on signal modula- 
tion, and thus that the recognition probability 
curves obtained without benefit of hydrophone 
sweep are applicable to the field situation. 


-- OBSERVED RECOGNITION 

DIFFERENTIALS 

Allowing for the difficulties and the differ- 
ences in procedure, the relations between 
primaudible signal and background spectra de- 
duced from these tests seem to be in substantial 
agreement wdth the trends discussed in Sec- 
tions 4.1 and 4.2. Three general types of 
primaudible component were detected: tones, 
propeller sounds, and distributed noises with 
little distinctive character. The results obtained 
for each type are discussed in this section. 
Some observations concerning the effects of 
hydrophone characteristics and filters are also 
described in this connection. 

Tones primaudible at 1.5 and 3 kilocycles 
are represented in Figures 5 and 6. Since the 
critical bands at these frequencies are about 
as wide as the 50-cycle analyzing band, detec- 
tion of sustained tones would be expected when 
the levels of primaudible signal and back- 
ground spectra are very nearly equal (see Fig- 
ures 76 through 79 in Chapter 4). Instead, the 
relation between the spectra indicated in five 
typical cases (Figures 5 A and 5B, and 6A, 
6B, and 6C) corresponds on the average to 
a signal-to-noise ratio of —5 decibels in the 
optimal band. As pointed out later, these cases 
represent a fairly wide sampling of test con- 
ditions; hence, the mean deviation from the 
average ratio is fairly large (3 decibels). 

The average spacing of 5 decibels between 
signal and background spectra at their points 
of closest approach is probably due largely to 
the observed fact that the tones fluctuated by 
between zb 3 and zb 5 decibels from the mean 
levels shown in the figures (see Section 4.2.2) 
and that the primaudible signals were detected 
at their peak, rather than their mean levels. 
In addition, the individual spectra show the 


OBSERVED RECOGNITION DIFFERENTIALS 


147 



1000 10,000 
FREQUENCY IN CYCLES 


A. Signal was sonic noise from an S-class submarine 
proceeding at 240 rpm (5 knots) and periscope depth, 
consisting of cavitation noise with moderate propeller thrash 
and also fluctuating tones at about 500 and 1,500 cycles. 
RD for presentation band shown : — 13 decibels. Char- 
acter of primaudible signal: 1,500-cycle tone and propeller 
cavitation in middle frequency region. 


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B. Signal was sonic noise from a fleet-type submarine 
proceeding at 100 rpm (5 knots) and periscope depth, 
consisting of cavitation noise with weak propeller thrash 
and also a strong sustained whine between 1 and 3 kilo- 
cycles. RD for presentation band shown : — 14 decibels. 
Character of primaudible signal : whine and hiss. 



2 4681 2 4681 


1000 10,000 
frequency in cycles 


C. Signal was sonic noise from a surfaced fleet-type 
submarine proceeding at 170 rpm (12 knots), consisting of 
cavitation noise with definite propeller thrash and also low- 
frequency grinding sound and irregular thuds and clanks. 
RD for presentation band shown : — 1 decibel. Character 
of primaudible signal : grinding , sound and fluctuating 
broad-band hiss. 


Figure 5. Effect of signal spectrum on audibil- 
ity. Background was water noise and wave slap, 
with sea state 1. The listening band extended 
from 0.2 to 10 kilocycles; the hydrophone used 
was a 3-foot crystal line. The presentation 
method consisted of interrupting the signal while 
the hydrophone axis remained fixed on the pro- 
jector bearing. 

influence of presentation method. Thus, Figure 
5A, obtained by blinking the signal, shows a 
spacing of about 10 decibels at 1,500 cycles, 
while Figure 6, obtained by training the hydro- 
phone, but under conditions otherwise similar 


to those indicated in Figure 5A, shows a spac- 
ing of about 6 decibels at the same frequency. 
In other words, square-wave modulation of the 
signal improved recognition by about 4 deci- 
bels. It should be noted, however, that audi- 
tory motion was present under both conditions 
of test ; it was merely more drastic in one case 
than in the other. Effects produced by auditory 
motion are also discussed in the following sec- 
tion. 

The spacing of 7 decibels at 3 kilocycles 
shown in Figure 5B is in general agreement 


KESTRICTED 




148 


FIELD MEASUREMENTS ON MASKING OF TARGET SOUNDS 



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FREQUENCY IN CYCLES 



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FREQUENCY IN CYCLES 



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FREQUENCY IN CYCLES 


Figure 6. Audibility of sonic noise from an S-class submarine proceeding at 240 rpm (5 knots) and peri- 
scope depth, masked by water noise and wave slap (sea state 1). The hydrophone used was a 3-foot crystal 
line. The presentation method consisted of training the hydrophone through the projector bearing while 
the level of the projected signal was maintained constant; the listening band was restricted, as shown in 
the figures, by inserting high-pass filters. The recognition differentials refer to the indicated presentation 
band. Character of primaudible signal: propeller sounds, when 5.5-kilocycle high-pass filter was inserted; 
otherwise, 1.5-kilocycle tone. 


with the preceding discussion. The spacing at 
1.5 kilocycles indicated in Figures 6B and 6C 
(about 4 decibels and 0 decibel, respectively) 
is smaller than the spacing shown in Figure 
6A. The character of the sounds which were 
presented in the tests illustrated in Figures 
6B and 6C differed from those illustrated in 
Figure 6A in that insertion of the filters pro- 
duced a reduction in the relative level of the 
primaudible component and possibly some dis- 
tortion in the optimal frequency region, as well 
as limiting of the rate of sweep modulation 
by the time constant of the filter. Such effects 
would be expected to make detection more diffi- 
cult. 


It should be noted at this point that the sig- 
nals shown in Figures 5 and 6 were radiated 
in such a way as to emphasize their low-fre- 
quency content. They are, therefore, not di- 
rectly comparable with the signals shown in 
Figure 7, which were radiated with uniform 
frequency weighting and which consequently 
had the characteristic slope (in the water) of 
about —6 decibels per octave. This partially 
explains the observation, indicated in Figures 
5A and 7, that a different signal component 
became primaudible in two sets of tests in 
which the same signal recording was used. In 
addition, the hydrophones indicated in Figure 
7 have greater discrimination against high-fre- 



OBSERVED RECOGNITION DIFFERENTIALS 


149 


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8 - FOOT LINE SEA STATE 3 

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FREQUENCY IN CYCLES 


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1000 


10,000 


Figure 7. Audibility of sonic noise from an S-class submarine proceeding at 240 rpm (5 knots) and peri- 
scope depth, masked by water noise and wave slap (sea states 1 and 3). The hydrophones used were 4- 
and 8-foot magnetostriction lines. Presentation method consisted of training hydrophone through projec- 
tor bearing while the level of the projected signal was maintained constant; the listening band extended 
from 0.2 to 10 kilocycles, and recognition differentials apply to this band. Character of primaudible sig- 
nal: propeller sounds. 


quency background noise than the one de- 
scribed in Figure 5; this factor, also, tended 
to shift the frequency of the optimal compo- 
nent. 

The primaudible sounds indicated in Figure 
5C were distributed rather than tonal. In this 
case, recognition occurred when signal and 
background spectra were essentially coincident 
throughout the presentation band, despite the 
square-wave modulation imposed on the signal. 
In fact, the signal-to-noise ratio in the band 
between 200 and 300 cycles is about 7 decibels. 
It should be noted, however, that the slopes 
of the spectra presented to the ear, as indi- 
cated in Figures 5 and 7, had a general trend 


of about 0 decibel per octave over most of the 
listening band. With this type of presenta- 
tion, the frequencies near 200 cycles are usu- 
ally threshold-limited rather than masking- 
limited when the gain is set for a comfortable 
loudness level (see P^igure 79 in Chapter 4). 
Thus, the RD shown in Figure 5C could prob- 
ably have been reduced by increasing the over- 
all gain. For the same reason, 200 cycles is 
specified as the lower limit of the listening 
band used in these tests. 

When propeller sounds were detected, the 
signal-to-noise ratio in the optimal band aver- 
aged — 2 decibels for the five cases shown in 
Figures 6D and 7, and the mean deviation from 


RESTRICTED 


150 


FIELD MEASUREMENTS ON MASKING OF TARGET SOUNDS 


this average was 2 decibels. These values ap- 
ply to detection of only one signal component, 
which appears to have become primaudible as 
a band of frequencies heard in the neighbor- 
hood of 8 to 9 kilocycles, and which was de- 
scribed by the observers as a series of pulses 
of very high pitch. This component may rep- 
resent a bearing squeak ; in any case, the spec- 
tra in Figures 6D and 7 seem definitely to indi- 
cate that a marked prominence appeared in 
the high-frequency region of the radiated sig- 
nal, and may imply that such peaks occur in 
the sonic output of ships and submarines (see 
also Figures 29 and 38 in Chapter 4). That 
this prominence between 8 and 9 kilocycles was 
present in the underwater signal used in these 
tests and not due to a peak in the response of 
the test hydrophones is indicated by the fact 
that it recurs in the spectra obtained with 
three different test hydrophones, and further 
by the fact that it is absent from the back- 
ground spectra received with two of those hy- 
drophones. 

The available spectra do not indicate whether 
the strong component at about 8.5 kilocycles 
was a tonal or a distributed sound, but the 
fact that the signal-to-noise ratio in the optimal 
region was approximately 0 decibel at primau- 
dibility implies that this component was prob- 
ably distributed over a band about 500 cycles 
wide, which is the width of a critical band at 
8.5 kilocycles (see the discussion of Figures 
29 and 38 in Chapter 4). It will be noted too 
that, within the optimal band, the signal-to- 
noise ratio required for primaudibility of this 
component was smallest (most favorable to de- 
tection) for the least directional of the three 
test hydrophones (Figure 6D) and largest for 
the most directional (compare Figures 7A and 
7C, or 7B and 7D). Examination of the data 
suggests that this deterioration may be asso- 
ciated with the increased peakiness of back- 
ground encountered in tests with the more di- 
rectional receivers (see Figure 3). Similarly 
a higher signal-to-noise ratio was required with 
the higher sea state (compare Figures 7 A and 
7B, or 7C and 7D) . 

One further precaution must be kept in mind 
when evaluating these tests in terms of prac- 
tical hydrophone performance. When the level 


and character of background are reasonably 
constant, such comparisons between the per- 
formance of test hydrophones can be made by 
noting the power which must be dissipated in 
the projector circuit in order to assure detec- 
tion with each hydrophone in turn. The over- 
all signal-to-noise ratios measured at the out- 
puts of the various hydrophones are not them- 
selves an adequate index of comparative hydro- 
phone performance since the level of the re- 
ceived noise may be considerably modified by 
the overall noise discrimination of a receiver, 
and there may nevertheless be only a very small 
effect on the level of the primaudible signal if 
the latter is detected at a frequency where hy- 
drophone directivity is poor. In the field, such 
a hydrophone would give about the same per- 
formance as a completely nondirectional re- 
ceiver, although the recognition differentials 
measured at the hydrophone outputs would be 
quite different in the two cases. It follows that 
the recognition differentials shown in Figures 
5 through 7 cannot be used directly for evalu- 
ating practical performance. 

The preceding analysis indicates that a vari- 
ety of factors, such as discrimination against 
noise, auditory motion, ease of train, coupling, 
system noise and band width, time patterns in 
the received signal and background, frequency 
of the primaudible component, and occasionally 
others, may be expected to affect the minimum 
signal level detectable with a given hydro- 
phone in a particular situation. A number of 
tests conducted during the program under 
discussion clearly reveals the desirability of 
using hydrophones with a large degree of dis- 
crimination against isotropic noise. However, 
since no adequate study of the effects of these 
factors could be undertaken during this test 
program, it is impossible to give any more de- 
tailed conclusions concerning comparative per- 
formance or optimal design characteristics on 
the basis of these findings. An examination 
of the effects to be expected from auditory 
motion is given in the next section. 

It should be mentioned in conclusion that the 
filter tests described in the present study con- 
firm (for a quasi-field situation, and for sounds 
presented to the ear with a slope of approxi- 
mately 0 decibel per octave) several of the 



AUDITORY MOTION 


151 


observations described in Section 4.1.8. Thus, 
it was found that narrowing the listening band 
by insertion of high-pass filters never made it 
possible to detect a given signal at a lower 
level. However, when the optimal component 
of that signal was not contained in the excluded 
portion of the spectrum, the use of filters made 
little difference. Occasionally, too, the primau- 
dible component seems to have been detected 
below the nominal low-frequency cutoff of the 
filter (see Figure 6C), although at a less favor- 
able signal-to-noise ratio. Finally, some ob- 
servations should be mentioned which were 
made on an occasion when successive rain 
squalls and heavy seas briefly and repeatedly 
caused the level of background to rise and 
fall by 10 to 15 decibels. In this case, the ob- 
servers did not themselves control the listen- 
ing hydrophone, and use of a high-pass filter 
eliminated the more characteristic features of 
the ship signal. It was found that the observ- 
ers could not detect the signal even when its 
spectrum was some 15 decibels above noise 
over the entire presentation band. They did 
hear changes of loudness in successive presen- 
tations but could attach no significance to them, 
owing to the absence of signal character and 
the high variability of background noise level. 


AUDITORY MOTION 

When a hydrophone can be trained back and 
forth across the target, the change in signal 
level constitutes auditory motion and may fa- 
cilitate recognition. Comparisons based on tests 
made under fairly similar background condi- 
tions indicate the general importance of this 
effect. The reference datum in these compari- 
sons is the masked threshold found when both 
hydrophone and mean signal level were held 
fixed. 

“Blinking” the signal, while the hydrophone 
was stationary, helped most. The magnitude 
of this effect, which has no counterpart in 
practical listening, does not appear to exceed 
4 to 5 decibels when the observers detect the 
same component in both conditions of test. 
About an equal degree of improvement was 
found when a highly directional hydrophone 


(4-foot or 8-foot line) was trained through 
the projector bearing at the optimal rate while 
mean signal level was held fixed ; less improve- 
ment resulted when a hydrophone with poor 
directivity was trained through the target. The 
other presentation methods gave intermediate 
degrees of improvement. Signals of gradually 
changing level could be detected 1 to 2 decibels 
further below noise when they were initially 
audible than when their intensities were raised 
from below the primaudible level (see Section 
4.2.6). Furthermore, this appeared to be true 
with trained as well as fixed hydrophones. 
Changes in the quality of the received signal- 
background mixture (associated with motion of 
the hydrophone) were easily heard when the 
signal was above its primaudible level, but such 
changes were almost imperceptible at primau- 
dibility.^® 

Since the presentation and scoring techniques 
used in these tests are not strictly comparable 
with those described in Section 4.2.1, it is im- 
possible to determine whether the reference 
datum mentioned above corresponds exactly 
to a 50 per cent RD as determined by the 
random-order method. If the shape of the 
transition curve changes markedly as a result 
of auditory motion, the change in RD for 95 
per cent recognition, for example, may be quite 
different from the change in the 50 per cent 
RD. It seems unlikely, however, that this 
datum involves a larger signal-to-noise ratio 
than is needed for 80 per cent detection proba- 
bility in the laboratory. Since the difference be- 
tween the 50 per cent and 80 per cent RD will 
not change by more than 1 to 2 decibels with 
changing conditions, it may be inferred that 
the net improvement due to optimal auditory 
motion is probably not less than 2 to 3 decibels 
(see also Table 1) . 

It should be noted that all the results quoted 
above apply to the situation in which the same 
component was detected in the modulated and 
unmodulated signals. In at least two cases, 
however, it seems that a relatively strong com- 
ponent, which the observers failed to detect in 
the unmodulated signals, became the primau- 
dible cue when modulation was introduced. In 
each of these cases, the component detected in 
the modulated sound permitted recognition of 


152 


FIELD MEASUREMENTS ON MASKING OF TARGET SOUNDS 


a fainter signal, and the recognition differen- 
tials were diminished by 10 to 12 decibels. The 
pertinent data will be found in reference 8.** 

Similar observations are described in Section 
4.2.2 ; listeners who failed to detect the optimal 
signal component required 6 to 8 decibels more 
gain in the signal channel than required by 
comparable observers. Thus, hydrophone sweep 
may compensate for the effects of inexperience 
or inattentiveness and, by directing attention 
to the optimal component, may occasionally 
double or triple the range at which a given 
operator can detect a given target. 

All the observations discussed down to this 
point in the present section were made with 

The effect of blinking: the signal, using a relatively 
nondirectional hydrophone, is indicated in Table IVa, 
line 1, of that report; similarly, the effect of hydro- 
phone sweep, using directional and nondirectional hy- 
drophones, is depicted on the right-hand side of Figure 
3 in the original report. See also page 17 and Table 
XII in that document. The second of these cases in- 
volved a change of hydrophone as well as of presenta- 
tion method; hence differences in system response or 
inherent noise may have played a larger part than did 
the presentation method. 


two signals of rather different character. One 
of these was recorded from a submerged S-class 
submarine and became primaudible as a fluctu- 
ating 1,500-cycle tone (see Figure 5A). The 
other was recorded from a freighter with four- 
bladed screws driven at 60 rpm; it contained 
a great deal of clanking sound and had a heav- 
ily accented propeller beat followed by three 
minor swishes. No analysis was made of the 
freighter signal, but comments of the observers 
indicated that the primaudible component had 
a frequency in the region of 1 to 2 kilocycles. 
Since the degrees and rates of modulation of 
the optimal components in these signals are 
unknown, it is impossible to decide to what 
extent the improvement due to training the 
hydrophone is limited by modulations already 
existing in the radiated signal. 

These observations appear to warrant the 
view that ability to train a directional hydro- 
phone will improve performance under fleld 
conditions. However, the major advantage 
would probably be reduced variability of the 
operator’s response (see Section 4.2.6). 




RESTRICTED 


Chapter 6 

SUMMARY OF LISTENING STUDIES 


« 1 BASIC FACTORS 

A PARTICULAR UNDERWATER sound or signal 
can be heard only if it is sufficiently intense 
compared with the background of unwanted 
sounds which tend to mask the signal. The sig- 
nal level which permits recognition in half the 
trials is called the signed ^recognition level, mea- 
sured in decibels. The difference between the 
signal recognition level and the background 
level is called the recognition differential. Since 
an increase in background noise requires a 
roughly similar increase in the signal recog- 
nition level, the recognition differential RD 
changes only slightly as the overall level of the 
presented sounds changes. The RD depends not 
only on the nature of the signal and back- 
ground presented to the ear, but also on the 
method used for measuring signal and back- 
ground levels. In particular the band widths 
used for such measurements must always be 
specified. 

Signal Characteristics 

The following characteristics of target 
sounds are significant in listening detection. 

Spectrum 

The measured sound power per cycle may 
show sharp peaks at certain frequencies, as a 
result of pure tones produced by vibrations of 
machinery on board ships. The sound from 
propeller cavitation shows a smooth spectrum, 
with the sound power per cycle present in the 
water dropping off by about 6 decibels every 
time the frequency is doubled. At considerable 
ranges the spectrum is modified by the change 
of transmission loss with frequency. Different 
types of targets may show characteristically 
different spectra. 


Modulation 

Machinery sounds may show rhythmic oscil- 
lation. Propeller sounds are generally modu- 
lated at the shaft r^ate, the number of revolu- 
tions per minute (rpm) ; at the blade rate, 
which equals the shaft rate times number of 
blades on each propeller ; or both. If directional 
listening gear is used to sweep across the tar- 
get, modulation can be imposed on an otherwise 
steady sound. 


^ ^ Background Characteristics 

Airborne noise may, if sufficiently intense, 
mask an underwater sound signal. This noise 
should always be reduced below the level of the 
background arising in the water, when the 
latter is presented to the ear at a comfortable 
listening level. Since airborne noise is usually 
most intense at the low sonic frequencies, a 
greater signal intensity is required at low 
frequencies than at high frequencies. 

Electrical noise may also mask a signal. With 
proper design and maintenance of the amplify- 
ing equipment, such noise can be made unim- 
portant as long as a listening hydrophone is 
used whose minimum measurable pressure is 
below the pressure of the noise background 
present in the water. This is usually feasible, 
except possibly at high supersonic frequencies 
under quiet listening conditions. 

The background of noise in the water may be 
reduced in importance by reliance on hydro- 
phone directivity. The acoustically significant 
properties of such noise are as follows. 


Spectrum 

Peaks at certain frequencies are not frequent 
in background spectra and are generally unim- 
portant when present. The sound power per 
cycle in the water falls off about 6 decibels each 


153 


154 


SUMMARY OF LISTENING STUDIES 


time the frequency is doubled, in much the 
same way as propeller cavitation, but increase 
of hydrophone directivity with frequency gives 
an effectively greater slope for the presented 
background spectrum than for the signal. 

Modulation 

The noise produced by the propellers of the 
listening ship shows marked modulation. Some 
noise shows marked peakiness, the peak factor 
increasing as increasing directivity or decreas- 
ing distance reduces the number of the indi- 
vidual noise sources contributing. 

Signal-Background Mixture 

The mixture of signal and background can 
be sharply modified by the receiving gear. The 
overall gain should be chosen to give a com- 
fortable listening level (loudness level of about 
70 decibels). Since signal recognition may 
occur at different frequencies for different tar- 
gets, a wide-band system is most effective. Fre- 
quencies above 10 kilocycles may be heard most 
effectively if they are heterodyned down to 
sonic frequencies. 

Some small advantage, especially in ease of 
concentration, is probably gained by use of a 
circuit which modifies the background spec- 
trum to give equal loudness in a critical band 
at each frequency. In such a spectrum the 
sound power per cycle drops about 3 decibels 
per octave from 0.1 to 2 kilocycles, and is then 
constant up to about 6 kilocycles, rising at even 
higher frequencies. Limiting probably does not 
appreciably affect the signal recognition level 
but may make target identification more diffi- 
cult. 

62 RECOGNITION LEVELS 

Steady Sounds 

The masking of steady sounds is simplest 
when the noise background presented to the ear 
has a smooth spectrum, which falls off by not 
more than 20 decibels for each doubling of 
the frequency. Such a steady sound can be 
heard 50 per cent of the time at a particular 


frequency, when the signal level in one of the 
ear’s critical bands centered at that frequency 
is just equal to the noise level in the same band. 
Thus a tone of frequency /o between 400 and 
1,000 cycles can be heard when its sound power 
is equal to the noise power in a 50-cycle band 
centered at f^. 

The recognition level of a signal is that level 
at which the signal can be heard in at least 
one critical band. The recognition differential 
for the signal is then 0 decibel, if the signal 
and background are each measured in the criti- 
cal band at which recognition occurs. If the 
signal and the background have parallel spec- 
tra, the RD is zero for any band used to mea- 
sure the signal and noise. In other cases, the 
RD may be a large negative number if the 
signal and background are measured in a wide 
band. If the noise background has an irregular 
spectrum or falls off more rapidly than 20 
decibels per octave, remote masking may occur, 
and sounds in one critical band may be masked 
by noise in another. This effect depends on the 
loudness level of the presented sound. Figure 6 
in Chapter 2 should be used in this case. 

Modulated Sounds 

If a single-frequency peak fluctuates slowly 
(less rapidly than about 5 times per second) 
and if the noise has a gently sloping spectrum, 
the RD as measured in a critical band is still 
0 decibel, provided that the peak level of the 
sound is measured. Modulation serves to call 
attention to signal components that might 
otherwise not be noticed ; thus modulation pro- 
duced by rotating a directional hydrophone 
helps to ensure most effective listening. 

Modulated sounds whose spectra are parallel 
to that of the presented background and which 
are first heard in several critical bands simul- 
taneously can be heard even when the peak 
intensity is below that of the background. The 
modulation may be inherent in the signal or 
may be imposed by rotation of a directional 
hydrophone. Provided that the modulation rate 
is between 1 and 10 per second, such a signal 
can be heard in half the trials if the total modu- 
lation of the signal-background mixture is 1 
decibel. Thus if the peak signal intensity is used 
for computing the recognition level and mea- 


TARGET IDENTIFICATION 


155 


surements are made in a band over which the 
noise and signal spectra are parallel, the RD 
for a wide-band sound modulated 100 per cent 
is —6 decibels. For 50 per cent modulation 
the corresponding RD increases to —5.4 deci- 
bels, while for 30 per cent modulation it is 
— 4.1 decibels. 


Transition Curves 

A plot of signal recognition probability 
against signal level is a transition curve. Such 
a curve shows not only the signal recognition 
level, defined for 50 per cent recognition, but 
also the levels required for other probabilities 
of recognition. The spread of such a curve is 
defined as the increase in signal level required 
to increase the recognition probability from 20 
per cent to 80 per cent. 

Variations in the spread from 1.3 to 8 deci- 
bels are observed. The spread tends to be less 


for cautious observers, but other changes in 
conditions have generally no systematic effect 
on the observed values of the spread. 


TARGET IDENTIFICATION 

Detailed identification of targets by their 
sound output may depend on the simultaneous 
perception of sounds at a number of different 
frequencies. Such perception may best be 
obtained by use of a wide-band listening sys- 
tem. Visual and mechanical methods are prob- 
ably inferior to the ear in target identification, 
since they lack the filter properties provided 
by the ear’s critical bands and since the quality 
of a sound cannot be simply recognized as yet 
by other than aural means.^ 

“ N.B. Useful concepts, whose meaning is devel- 
oped at diverse points in the foregoing and following 
chapters, have been indexed under unified headings to 
facilitate synopsis and review. 


RESTRICTED 


Chapter 7 


CHARACTERISTICS OF ECHOES AND REVERBERATION 


L istening is subject to the serious limita- 
tion that a submarine can, if it wishes, be 
extremely quiet, and practically impossible to 
hear more than a few hundred yards away. 
Thus for reliable antisubmarine detection it is 
necessary to send out a pulse of sound and 
listen for the returning echo. This technique 
has the added advantage that it gives the dis- 
tance, or range, of the submarine, information 
not readily obtained with underwater listen- 
ing. In echo ranging as in listening, however, 
the signal can be masked by the background of 
sounds reaching the sonar operator. Knowledge 
of the audibility of echoes may be helpful in 
choosing the type of pulse to be sent out as well 
as in the design of the receiving gear. Hence 
studies of echo masking have operational im- 
portance, and considerable research along these 
lines has been carried out. 

To understand the results obtained in this 
research on echo masking requires a knowledge 
of the basic properties of the signal and back- 
ground, since from a basic standpoint these 
properties determine the recognizability of the 
signal. Some of these relevant properties have 
already been discussed in Chapter 3. For ex- 
ample, the properties of a noise background 
have been outlined in some detail in Section 
3.2; since in many practical conditions noise 
may be the component of background which 
masks an echo, this previous discussion of noise 
is also relevant in echo masking studies. Simi- 
larly, the discussion of sonar gear given in Sec- 
tion 3.3 is also relevant here, since supersonic 
equipment used in echo ranging is frequently 
used also for supersonic listening. 

The present chapter is therefore devoted to 
a description of several properties of under- 
water sounds which have not yet been described 
and which are important in the masking prob- 
lem. First, in Section 7.1, some of the basic 
properties of short pulses are briefly discussed. 
The following section describes the quality of 
the echoes returned from various targets, and 
points out the amplitude and frequency prop- 


erties of such echoes, which can be important 
in attempts to distinguish echoes from back- 
ground. Finally, the properties of reverbera- 
tion, which may be the dominant component of 
background if the noise level is low, are dis- 
cussed in Section 7.3. 

7 1 IDEAL PULSES 

The basic sound used in echo ranging is a 
pulse of supersonic frequency. In practice, the 
upper limit on the length of the pulse is set by 
requirements of range accuracy and on the 
reverberation level, which increases with in- 
creasing pulse length. The lower limit is deter- 
mined by deterioration of echo strength with 
decreasing pulse length and by the increasing 
difficulty of recognizing the shorter pulses. The 
pulse is generated electrically and is trans- 
formed into sound by the projector (trans- 
ducer) . The emitted sound wave then travels to 
the target and is reflected back. On its return 
to the hydrophone, or transducer, it is detected 
by the same equipment which generated it. 
Usually it is heterodyned down to an audible 
frequency and detected aurally. A variety of 
effects can modify the sound as it passes 
through the water, is reflected by the target, 
and is converted by the receiving equipment 
into an audible sound. However, the basic prop- 
erty of the original transmitted pulse is one of 
the important factors which determines the 
physical properties of the perceived echo. The 
character of such a supersonic pulse is there- 
fore discussed in this chapter. 

Ideally, a pulse consists of a train of waves 
of constant amplitude and frequency lasting 
for the time r. While practical deviations from 
this ideal may exist and will be discussed later, 
analysis of this ideal pulse casts light on the 
physical situation. 

Spectrum 

While such an ideal pulse has a constant fre- 
quency during the limited time it is being gen- 


156 


[ESTRICTEll 


IDEAL PULSES 


157 


erated, its effect on a tuned receiving system is 
not that of a sustained single-frequency tone. 
As a result of the short duration of the pulse, 
attempts to analyze the sound with sharply 
tuned systems show that the pulse is similar to 
a band of sound spread over a range of fre- 
quencies. This may be viewed on one hand as 
an effect of the transients produced in a tuned 
system by a short pulse. A system sharply 
tuned at any frequency will show some re- 
sponse for a very brief input of a quite differ- 
ent frequency. Thus a pulse only 1 cycle in 
length will produce response in any acoustical 
or electrical filter tuned to any frequency 
within a broad band; in this particular sense 
the pulse is indistinguishable from a noise 
pulse. 

Another way of looking at the problem is to 
regard the short pulse as the sum of continuous 
sound waves whose amplitudes and phases are 
such that during the interval r they add up to 
the pulse and at all other times they cancel out. 
This collection of sustained tones, equivalent 
to the pulse, is called the spectrum of the pulse ; 
the amplitude of each tone of frequency / is 
denoted by A (/) . Each tone of amplitude A (/) 
is sometimes denoted as a spectral component 
of the pulse. This analysis is convenient in that 
the amplitude of A (f) gives immediately the 
response of a tuned filter of frequency / when 
the pulse is fed into it. 

The amplitude of the spectral component 
may be found by methods which are standard 
in the theory of Fourier series and Fourier 
integrals. The sound pressure p(t) in a short 
pulse of frequency /o is given by the expres- 
sion 


p{t) = po cos 2Trfd , (1) 

when the time t lies within the interval r. If we 
measure the time from the middle of the pulse, 
then equation (1) holds when t is greater than 
— t/2 and less than +r/2. At all other times 
p(t) vanishes. From this equation it may be 
shown that the amplitude of the Fourier com- 
ponent frequency / is given by 

r t /2 

^(/) =Po / cos 27r/(0 cos 2'KfMi . (2) 

J—z/2 

This expression may be integrated directly and 
gives the relationship 


A(f) = 


[sin TT (f + /n) T , sin t (f — /o)r \ 
2x(/ + /.) 2,r(/-/„) /• 


Several results are evident from this equation. 
In the first place, A (/) is greatest when / is 
almost equal to /q. Under these conditions the 
second term is very much greater than the first. 
The total energy in the spectral components 
present in a band 1 cycle wide is proportional 
to A^. Most of this energy is at those frequen- 
cies for which the second term in equation (3) 
is large. As / — /« increases, the second term 
vanishes when the argument of the sine func- 
tion is ±7r. Thus the value of / for which this 
term vanishes is given by 


(/ - /o)r = ± 1 . (4) 

The total width A/ of this spectral region of 
high energy is obviously twice the value of 
f — fo found from equation (4) . Thus 

Af = -. (5) 

r 

While the spectrum of the pulse given by equa- 
tion (3) extends to very large and very small 
values of the frequency, most of the energy of 
the pulse is included within the band whose 
width A/ is given by equation (5) . This width is 
therefore called the essential width of the pulse 
spectrum. Alternatively, the ^‘width” of the 
pulse spectrum is sometimes defined as the fre- 
quency separation of the two points where the 
spectrum level is 3 decibels down from its 
maximum value, between the points where 
AU/) is y^A^Uo)- On this definition. A/ is 
about equal to l/r. With this definition, the 
total energy in the pulse is just equal to the 
energy per cycle at the midfrequency /o, multi- 
plied by the essential width A/, or 1/t. In this 
report, however, equation (5) will be used to 
define the essential width a/ of the pulse 
spectrum. 

If certain of the spectral components of the 
pulse are suppressed, the resultant sum of the 
different tones will no longer exactly resemble 
the pulse and will, moreover, contain some 
sound energy at time intervals outside the 
limits of the pulse. However, if all the frequen- 
cies in the essential width of the spectrum are 
included and those outside are omitted, most of 
the energy in the pulse will still be present al- 


158 


CHARACTERISTICS OF ECHOES AND REVERBERATION 


though the shape of the pulse may be seriously 
distorted (see Figure 1 in Chapter 8). 

This result, that the essential width of a 
pulse spectrum is inversely proportional to the 
pulse duration, lies at the base of much of the 
study of short pulses. Thus, a pulse which lasts 
only 10 milliseconds has an essential width of 
200 cycles ; a band-pass filter which admits less 
than 200 cycles or whose midfrequency is not 
centered at the pulse frequency /o will distort 
and weaken the pulse very seriously. Also when 
the response of tuned systems is used to deter- 
mine the properties of such a short pulse, 100 
cycles give an order-of-magnitude upper limit 
to the accuracy with which the pulse frequency 
can be simply measured. A receiving system 
admitting a band only 1 cycle wide would show 
about the same response to the pulse when 
tuned anywhere within a band some 25 cycles 
wide centered at the frequency f^. To obtain 
very much greater accuracy with tuned sys- 
tems, it would be necessary to measure A (/) 
for a number of different values of / and to 
determine /o as the midfrequency of the ob- 
served spectrum. 

Another important result also follows from 
equation (3). This equation may be used to 
give the energy per cycle at frequencies outside 
the essential width of the spectrum. The energy 
per cycle is proportional to A^; outside the 
essential width of the spectrum the sine func- 
tions of equation (3) will oscillate between +1 
and —1, but on the average, the square of the 
sine equals V2> while the average value of the 
product of the two sines is zero. Thus if we 
average over bands which are wide compared 
with the essential width of the spectrum, but 
centered far away from the midfrequency of 
the spectrum, we obtain 


^ 2 (/) = ^ . ( 6 ) 
^ 4 x 2 (/2 - /„ 2)2 

From this equation it follows that when / is 
not too widely different from /o, the numerator 
in equation (6) is constant, and the average 
energy per cycle decreases inversely as 
(/— /o)^ For a frequency / much greater than 
/o, the average energy per cycle is proportional 
to 1/f-. In this situation the spectrum level 
decreases 6 decibels every time the frequency 
is doubled. It will be noted that this level is 
independent of the pulse length. Thus, even for 


a long pulse, energy will be present at frequen- 
cies far removed from the midfrequency /q. 
However, the total energy in the pulse in- 
creases directly with the pulse length, and thus 
the relative amount of the energy at such dis- 
tant frequencies decreases with increasing 
pulse length as expected. 

This presence of frequency components at 
frequencies far removed from the midfre- 
quency /o depends very critically on the abrupt 
beginning and termination of the assumed ideal 
pulse. When the pulse begins more gradually, 
building up to full amplitude in several cycles 
and falling off similarly at the end, the essen- 
tial width of the spectrum is not appreciably 
changed, but equation (3) can no longer be 
used to find the energy at frequencies far re- 
moved from /o. In fact, the energy present at 
such frequencies will be very much less for a 
rounded pulse than for an ideal square-topped 
pulse. 

The importance of these results, summarized 
in equations (5) and (6), is emphasized by the 
fact that the distribution of energy in the spec- 
trum of a pulse is not changed by most of the 
processes which modify the pulse as it travels 
out and is reflected, received, and finally recog- 
nized as an echo. In the first place, most of the 
phenomena affecting a pulse are linear; this 
means that their magnitude is independent of 
the absolute intensity of a pulse, and an emitted 
pulse with twice the original amplitude will be 
exactly the same as the original pulse when it 
is finally presented to the operator, except that 
its amplitude will be twice as great. As a result 
of this fact, each spectral component is un- 
affected by the presence of others and is trans- 
mitted and reflected in exactly the same way as 
would a single continuous tone of that fre- 
quency. In the second place, both the transmis- 
sion and the reflection of sound energy do not 
change much with frequency over the essential 
spectrum of a supersonic pulse, provided that 
this is not much less than a millisecond in dura- 
tion. As a result of these two conditions, the 
energy in each 1-cycle band in the spectrum of 
the echo returning to the transducer should be 
the same as the energy in each 1-cycle band in 
the spectrum of the transmitted pulse. The 
phases of different spectral components will be 
modified as the pulse is transmitted and re- 



IDEAL PULSES 


159 


fleeted, thus leading to distortion of the pulse, 
but the magnitude A^(f) should be unaffected 
by these processes. Thus in the returning echo, 
as in the pulse, the width of the essential spec- 
trum is given by equation (5), and for high 
frequencies the energy per cycle should de- 
crease 6 decibels for each doubling of the fre- 
quency. The receiving equipment if sharply 
tuned may modify the distribution of energy 
among the different spectral components and 
thus change the spectrum of the echo presented 
to the sound operator. If the hydrophones or 
loudspeaker used are not flat over the pulse 
spectrum, the distribution of energy over the 
spectrum may again be changed. These effects 
are usually important only when very short 
pulses are considered (less than 10 millisec- 
onds long), or when spectral components far 
from the midfrequency are important. 

The foregoing considerations have been de- 
veloped for a pulse with certain ideal proper- 
ties. Actual echo-ranging projectors do not 
usually generate such ideal pulses. A projector 
which is mechanically resonant, such as a mag- 
netostriction projector, is very inefficient for 
sustained tones whose frequencies are far from 
resonant. For this reason, if an ideal square- 
topped pulse is fed electrically into such a pro- 
jector, the distant-frequency components will 
not be radiated and the pulse will therefore 
become rounded, since these distant compo- 
nents are necessary to produce the abrupt be- 
ginning and ending of the pulse. This effect is 
identical with the distortion of the spectrum of 
the received echo by a tuned receiving system 
discussed earlier in this section. Other imper- 
fections in practical equipment may be ex- 
pected to produce some irregularities both in 
the amplitude and the frequency of the pulse, 
and thus to modify its spectrum from the 
simple form given in equation (3). All these 
possibilities must be kept in mind when the 
observed spectrum of a pulse is compared with 
theoretical expectations. 


Frequency Modulation 

The frequency properties of emitted pulses 
have been studied by means of the periodme- 
ter,i a device which measures the time interval 


between successive zeros in the sound pressure. 
This interval may be regarded as the period of 
oscillation of the wave. Thus, the instrument 
can be used to determine very accurately the 
instantaneous frequency of a sound wave, de- 
fined as the reciprocal of the measured period. 
If the successive periods are not equal, the 
periodmeter then gives information on the dis- 
tribution of these periods, that is, on the num- 
ber of intervals lying within each range of 
time. This information can be interpreted as 
giving the distribution of frequencies in the 
emitted pulse, since to each period there cor- 
responds an instantaneous frequency. This in- 
formation is quite different from that provided 
by an analysis of the pulse spectrum. For ex- 
ample, in an ideal pulse the emitted sound pres- 
sure is an exact cosine function, which equals 
zero at regular intervals and which, therefore, 
has a constant period. 

The periodmeter would show that such a 
pulse had a constant frequency for its entire 
duration. The spectrum, however, would show a 
spread of frequencies in accordance with equa- 



Figure 1. Periodmeter analyses of pulses. Hori- 
zontal lines indicate 20-cycle intervals. 


tion (3). While this difference between the re- 
sults of the periodmeter and the spectrum must 
be kept in mind, the periodmeter provides use- 



160 


CHARACTERISTICS OF ECHOES AND REVERBERATION 


ful information on the basic frequency proper- 
ties of sound waves. 

When the periodmeter is used to analyze 
pulses sent out by echo-ranging gear, variation 
in the frequency of the emitted sound is often 
discovered. Typical examples are shown in Fig- 
ure 1, where periodmeter records are shown for 
pulses produced by special laboratory electronic 
gear at University of California Division of 
War Research [UCDWR] (lower record) and 
by standard Navy sonar gear on board one of 
the West Coast Sound School ships at San Diego 
(upper record). The height of each vertical 
white line represents the interval between two 
successive zeros in the sound pressure. Since 


verberation but is garbled by chattering in the 
change-over relay. The durations of the pulses 
shown were about 50 milliseconds. These rec- 
ords were made from the signal input to the 
transducer, but similar results were obtained 
from the received acoustic signal. 

The pulse obtained with the high quality 
equipment shows no frequency modulation, but 
the pulse produced by the standard gear shows 
a variation of frequency over a range exceed- 
ing 40 cycles. Although this variation is not 
large, it could be important in some situations. 
Especially for longer pulses, for which the es- 
sential width of the spectrum is narrow, the 
presence of frequency modulation in the actual 



|<- P1N6 — ^ 



PING — »j 



NO DOPPLER 



h— echo-H 

UP DOPPLER 


Figure 2. Periodmeter analyses of pulses and echoes. Horizontal lines indicate 20-cycle intervals. 


the scale is nonlinear, horizontal guide lines 
have been drawn at intervals corresponding to 
20 cycles in the instantaneous frequency. The 
record to the right of each ping represents re- 


pulse can significantly broaden the spectrum 
of both the pulse and the echo (see Figure 2). 
This effect is particularly important in the ac- 
curate determination of doppler shifts. 



ECHOES 


161 


l/2'MILLISECOND PULSE ECHO 



SUCCESSIVE ECHOES 


lO-MILLISECOND PULSE ECHO 




Figure 3. Echoes from a submarine at beam aspect. 


72 ECHOES 

An echo can, in general, be much more com- 
plicated than the pulse which produces it. The 


sound may become distorted during transmis- 
sion through the water, especially in shallow 
water where multiple paths tend to prolong 
the signal received at the target. In addition. 






162 


CHARACTERISTICS OF ECHOES AND REVERBERATION 


the sound is distorted by reflection from the 
target. This second effect is usually more im- 
portant than the first, and will be given chief 
attention here. The amplitude and frequency 
characteristics of observed echoes are discussed 
in the following two sections. 


Amplitude 

A small geometrically smooth target, such 
as a sphere or triplane, reflects the sound specu- 
larly back toward the echo-ranging projector. 
Such specularly reflected sound is generally a 
relatively exact reproduction of the received 
pulse. In practice, the echo from a mine case 
may be of this character. Echoes from large 
targets, however, such as surface vessels and 
submarines, are generally the sum of echoes 
from many reflectors. While the echo from 
each reflecting surface may reproduce the 
pulse, the sum of the component echoes some- 
times bears relatively little resemblance to the 
pulse. For example, the phases of different 
component echoes will interfere constructively 
with each other, while in other cases, the inter- 
ference will be destructive. Thus, in general, 
the total received echo may be considerably 
prolonged from the original short pulse and 
may contain considerably more amplitude mod- 
ulation, resulting from interference between 
the different component echoes making up the 
total echo. 

The simplest echoes from large targets are 
those received from a ship at beam aspect. For 
a submerged submarine at beam aspect, for 
example, the echo comes primarily from specu- 
lar reflection produced by the cylindrical sides 
of the submarine, from the pressure hull and 
ballast tanks. This echo, like that from a 
sphere, tends to be a fairly accurate reproduc- 
tion of the pulse, as is evident from the traces 
shown in Figure 3. It is evident from the re- 
production for a short pulse shown in this 
figure that even these beam echoes show some 
structure. Thus several reflecting surfaces pro- 
duce the observed beam echo.^ For surface 
vessels, the available evidence is not complete 
but indicates that the echo returned at beam 
aspect tends to be a fairly accurate reproduc- 


tion of the outgoing pulse. Such an echo, which 
like the pulse is approximately square-topped, 
is sometimes known as a clean echo, in contrast 
to the smear echo discussed in the next para- 
graph. 

Observations on submerged submarines show 
that specular reflection from the sides of the 
submarine begins to weaken when the axis of 
the submarine is not exactly perpendicular to 
the sound ray. When the submarine aspect dif- 
fers by more than 15 degrees from the exact 
beam, this specularly reflected sound presum- 
ably goes off in another direction and is not 
detected back at the echo ranging vessel. The 
echo which is observed at these off-beam as- 
pects is apparently produced by reflecting and 
scattering surfaces distributed over the entire 
length of the submarine. Thus, the echo will 
be considerably prolonged over the pulse dura- 
tion r by an amount depending on the length of 
the submarine in the line of sight. The ob- 
served duration of the echo, in seconds, is equal 
numerically to r+ (L cos 0) /2c, where L is the 
length of the submarine, 0 is the angle between 
the submarine axis and the direction of the 
sound ray, and c is the velocity of sound.^ Such 
an echo, which is prolonged relative to the 
pulse and whose envelope is jagged and uneven, 
has been called a smear echo in contrast to the 
clean echo discussed previously. The average 
intensity in a smear echo would be expected to 
decrease with decreasing t since the total en- 
ergy of the pulse, and therefore of the echo, 
must decrease as r decreases. It has been ob- 
served experimentally that the peak intensity 
of a smear echo tends to decrease with r espe- 
cially when T is less than 10 milliseconds.^ 

Sample smear echoes for different pulse 
lengths are shown in Figure 4. All these were 
obtained from a submarine at off-beam aspects. 
It will be noted that the structure of these off- 
beam echoes begins to resemble that of rever- 
beration, discussed in the following section. In 
particular, the length of each blob observed in 
the echo is roughly the same as the length of 
the emitted pulse. It is uncertain whether the 
heights of the different blobs observed in these 
smear echoes are correlated with different re- 

“ For a more detailed discussion see Division 6, Vol- 
ume 8, Chapters 23 and 24. 


( RESTRICTED J 


ECHOES 


163 




ASPECT ANGLE 
IN DEGREES 

0 


180 


220 


QUARTER 


345 


QUARTER 


Figure 4. Echoes from a submarine at off-beam aspects. 


164 


CHARACTERISTICS OF ECHOES AND REVERBERATION 


fleeting surfaces on the submarine or whether 
they are entirely the result of random inter- 
ference between all the different scatterers. 
The extent to which the structure of a smear 
echo tends to repeat itself from one echo to the 
next is therefore uncertain. However, there is 
no question but that the time-amplitude pattern 
of such a smear echo tends to be irregular, with 
the blob size roughly equal in length to the 
pulse duration. While detailed investigations 
have been carried out only for submarines, 
these same conclusions are probably valid for 
any type of large target. 


Frequency 

The frequency properties of the returned 
echo are partly determined by the frequency 
properties of the pulse. In particular, it has 
already been noted that the energy per cycle 
in the spectrum of the returning echo is almost 
exactly the same as that in the spectrum of 
the emitted pulse. If the target is in relative 
motion, the entire spectrum of the echo will 
be shifted by the frequency {2v/c)f,„ where 
V is the motion of the target in the direction 
of the incident and reflected sound, c is again 
the velocity of sound, and /o is the midfre- 
quency of the pulse. This doppler shift simply 
translates the entire spectrum by a uniform 
amount, however, and does not otherwise 
change the signal. Motion of the echo-ranging 
projector through the water also produces a 
doppler shift in both the emitted signal and 
the received echo. These shifts are not pri- 
marily important in masking studies, however, 
since the doppler shift of interest is that of 
the echo relative to the reverberation, which is 
given by the preceding formula. 

While the distribution of energy in the echo 
spectrum is identical with that of the pulse, 
apart from a uniform doppler shift, the relative 
phases of the different spectral components 
may be considerably modified by interference 
between the different component echoes mak- 
ing up an observed smear echo. These inter- 
ference effects in a smear echo can affect the 
instantaneous period of the sound. Results ob- 
tained by the periodmeter show that smear 


echoes can possess a considerable amount of 
frequency modulation. Since the factors gov- 
erning the response of the ear to complex and 
variable sounds are not yet well understood, 
it is uncertain whether the distribution of in- 
stantaneous frequencies shown by the period- 
meter or the different frequency components 
shown by the spectra correspond most closely 
to the properties perceived by the ear. It is 
possible that both of these methods of describ- 
ing echoes must be taken into account in a 
realistic explanation of how the ear recognizes 
echoes. 

Echoes can also be obtained with pulses of 
frequency-modulated sound. One situation of 
practical importance is that in which the emit- 
ted pulse has a constantly ascending frequency. 
As with a pulse of constant-frequency (CW 
pulse), a frequency-modulated (FM) pulse 
striking a vessel at beam aspect gives an echo 
which tends to reproduce the outgoing pulse. 
For example, echoes from a submarine at beam 
aspect, obtained with the pulse shown in Fig- 
ure 2, show exactly the same frequency modu- 
lation as that in the original pulse. Off-beam 
echoes, on the other hand, will again involve 
the superposition of the different component 
echoes from the different reflecting surfaces. 
With FM pulses, this combination of compo- 
nent echoes from different ranges will com- 
bine sounds of different instantaneous fre- 
quencies, and the resultant echo may be ex- 
pected to have rather complicated characteris- 
tics. These effects have not been studied in 
detail, but may be expected to affect the per- 
formance of the ear in recognizing such echoes. 
In addition, the intensity of echoes produced 
with an FM pulse seems to show somewhat less 
variability from echo to echo than is found 
with CW echoes. 


73 REVERBERATION 

If the ocean were perfectly uniform and the 
surface and bottom were smooth and flat, the 
only sound reflected back to the echo-ranging 
projector would be that from the target, and 
the noise level would be the only source of 
masking. In fact, however, the ocean is not 


^( restrict ed I 


REVERBERATION 


165 


uniform and the ocean surface and bottom are 
not perfectly smooth and flat. As a result, 
sound is scattered from the many inhomog- 
eneities in the body of the ocean and from 
the irregularities at the surface and bottom. 
This scattered sound is heard back at the echo- 
ranging installation as a rolling sound known 
as reverberation. At close range the reverbera- 
tion level is high and will usually lie so far 
above noise level that it forms the dominant 
part of the background against which an echo 
at short range must be recognized. Even at 
longer ranges reverberation under some con- 
ditions, as for instance, over a shallow rocky 


Secondly, the available evidence on the rever- 
beration from a frequency - modulated pulse 
(FM reverberation) is briefly summarized. 


^ GW Reverberation 

The reverberation received from a CW pulse 
has approximately the same frequency as the 
outgoing pulse. However, its amplitude is mod- 
ulated in an irregular way, and a closer exami- 
nation of its frequency pattern shows irregu- 
larities there also. The amplitude properties 
of reverberation are discussed in the next sec- 



< 



Figure 5. Time-amplitude-pattern of CW reverberation showing the variability in reverberation received 
from successive transmissions under nearly identical conditions. The writing speed was higher for the 
shorter pulse. For a better indication of the relative blob durations see Figure 7 in Chapter 9, 


bottom can be the important component of 
the background. The reverberation intensity is 
proportional to the total sound power radiated 
into the water, and thus increases proportion- 
ally with the pulse length, as long as the pulse 
length, in yards, is much less than the range 
at which the reverberation is measured. 

Since reverberation is the sum of the indi- 
vidual echoes returned by myriads of scatter- 
ers, it is very similar in its properties to the 
smear echoes discussed in the preceding sec- 
tion.’^ Those features which are important to 
the masking problem are summarized here. 
First, the characteristics of reverberation pro- 
duced by a single-frequency, or continuous- 
wave pulse (CW reverberation) are discussed. 

*’ The detailed properties of reverberation are dis- 
cussed in Division 6, Volume 8, Chapters 11 through 17. 


tion, while the frequency properties are treated 
later. 

Amplitude Modulation 

The amplitude variation of CW reverbera- 
tion is very marked. Typical power level rec- 
ords of reverberation generated by different 
pulse lengths are reproduced in Figure 5. 
These were made from an oscillograph re- 
sponding directly to the received reverberation 
at 24 kilocycles before heterodyning. It was 
shown by Lord Rayleigh many years ago that, 
when sound is received from many scatterers, 
the probability P{I) of receiving an intensity 
greater than I is given by 

P(7) = , (7) 

where Iq is the average intensity received. 


166 


CHARACTERISTICS OF ECHOES AND REVERBERATION 


This distribution of intensity is known as the 
Rayleigh distribution. Equation (7) may be de- 
rived from the consideration of the random 
phases of the scattered sound.'" In the case of 
reverberation, the intensity Iq is itself a func- 
tion of time and must be found by measure- 
ments over a great many successive reverbera- 
tions. In each reverberation the intensity must 
be measured at a fixed time interval t after 
the pulse has been emitted. The average of all 
these intensities over many successive rever- 
berations is then called the average intensity 
at the time t. The probability of deviations 
from this average value is given by equation 
(7). This equation has been confirmed experi- 
mentally for CW reverberation and may be as- 
sumed to be correct for all frequencies of prac- 
tical interest. While these studies were carried 
out for the unheterodyned reverberation, the 
same result would presumably apply to the het- 
erodyned reverberation also. 

The blob size in the unheterodyned rever- 
beration shown in Figure 5 is roughly equal 
to the length of the emitted pulse, a result 
which is also in agreement with theoretical 
expectations. 

Decay Rate 

The rate at which reverberation decays can 
have an important infiuence on the problem of 
echo recognition. It is customary to plot a de- 
cay curve for reverberation showing how the 
average reverberation changes with time 
elapsed since the emission of the pulse. In 
inspecting such a curve, it must be borne in 
mind that deviations from the mean intensity 
have the probability given in equation (7) and 
that actual reverberation is consequently much 
more jagged and irregular than is shown by 
the mean curve. The rate at which the average 
reverberation decays is found to be a highly 
variable quantity. Sometimes the reverbera- 
tion falls away rapidly to a very low level; 
sometimes it remains at a sustained level for 
several seconds and then gradually diminishes. 
In shallow water, for example, when tempera- 
ture gradients bend a sound beam downward, 
the reflected sound received from the bottom 

See Division 6, Volume 8, Chapter 7. 


appears quite suddenly and for a relatively 
short time produces a crash of reverberation 
which may, under some conditions, be mistaken 
for an echo. This variability of the decay rate 
of reverberation, as oceanographic factors 
vary, makes it difficult to devise automatic 
level stabilizers which will keep the background 
at a constant level without smoothing out the 
echo. 

Frequency Properties 

One of the most noticeable characteristics of 
CW reverberation is its tonality. Since rever- 
beration is simply the sum of many echoes, the 
distribution of the energy in its spectrum is the 
same as that in the spectrum of the emitted 
pulse. The scattering centers are usually not 
moving through the water at any high speed; 
therefore, reverberation usually has no doppler 
except that produced by the motion of the echo- 
ranging projector or receiver. The only excep- 
tion to the latter statement is encountered in 
the case of strong currents, which produce sys- 
tematic shifts between the reverberation from 
the water and the reverberation from the bot- 
tom. Because water reverberation is likely to 
come in at close range and bottom reverbera- 
tion at longer range, a marked and systematic 
shift of reverberation frequency with range 
may be produced. Such effects will, of course, 
shift the entire spectrum without modifying 
the relative amplitudes of the spectral com- 
ponents. 

Motion of the echo-ranging projector can 
produce marked doppler shifts in the received 
reverberation. Since these frequency shifts 
vary with relative bearing, and since the 
projector has an appreciable beam width, the 
spectrum of reverberation may be appreciably 
broadened by the motion of the echo ranging 
vessel. A detailed discussion of this effect is 
postponed to Section 10.12, and in the present 
description only the reverberation received by 
a stationary transducer is considered. 

An analysis of the spectrum of the rever- 
beration obtained with a stationary projector 
was made by the British early in the war. By 
use of a narrow-band filter, the reverberation 
energy per cycle at various frequencies was 


{restricte^ 


REVERBERATION 


167 


determined. This energy distribution was 
found to be identical with that measured for 
the pulse, in agreement with expectation. How- 
ever, a similar investigation at UCDWR, 
making use of the periodmeter, yielded results 
not in agreement with theory.^ Further ob- 
servations are therefore required before the 
theory can be regarded as confirmed. 

It will be recalled that the periodmeter gives 
information on the instantaneous period of the 
reverberation, which is not directly related to 
the spectrum. A typical result is shown in Fig- 
ure 6, which is a reproduction of a periodmeter 
record for reverberation. As before, the height 
of each vertical white line in each of the dia- 
grams represents the time interval between 
successive zeros in the heterodyned reverbera- 
tion; the corresponding frequency scale is in- 
dicated by the horizontal white lines. 

The pulse producing these reverberations 
showed very little frequency modulation, since 
laboratory equipment was used to produce a 



25-MILLiSECOND PULSE 



92- MILLISECOND PULSE 


Figure 6. Periodmeter analyses of volume rever- 
beration. Horizontal lines indicate 20-cycle 
intervals. 

pulse similar to that shown in the upper record 
of Figure 1. Nevertheless it is evident from 
Figure 6 that the distribution of zeros in each 
reverberation sample is highly irregular, with 


rapid fluctuations from one period to another. 
An analysis of other records shows that for 
reverberation from a 92-millisecond pulse the 
root mean square deviation of periods is about 
2 per cent of the average period, corresponding 
to a frequency deviation of 16 cycles at 800 
cycles. While the reverberation was obtained 
from a projector moving at 8.5 knots, it was 
found that this ship speed was too low to pro- 
duce any marked effect on the observed spread. 
The corresponding spread found for the out- 
going pulse is, of course, very much less, and 
for an ideal pulse, with no frequency modula- 
tion, would be zero. 

This spread of instantaneous periods in the 
received CW reverberation is the direct result 
of the severe amplitude modulation of the rever- 
beration. While the detailed nature of the 
periodmeter results has not been explained, it 
seems clear that the interference of echoes from 
many targets produces a sound which in effect 
has rapid frequency modulation as well as am- 
plitude modulation. This spread of instantane- 
ous frequency, as well as the width of the es- 
sential spectrum, probably affects the perform- 
ance of the ear when presented with these 
sounds. 

Systematic changes of reverberation fre- 
quency with time have also been observed with 
the periodmeter. The observations indicate that 
the reverberation which arrives immediately 
after the pulse is sent out may have a higher 
or lower frequency than the reverberation 
arriving several seconds later. It has already 
been pointed out that some such effect would 
be expected in the presence of variable water 
currents, especially in shallow water. However, 
a steadily rising or falling pitch in reverbera- 
tion seems to occur more frequently than can 
be explained on this basis. Possibly these 
changes in pitch are simply the result of statis- 
tical fluctuations. In any event, systematic fre- 
quency changes during the arrival of rever- 
beration provide one more factor that may in- 
fluence the masking effect of reverberation on 
echoes. 

Heterodyne Frequency 

For aural recognition, the reverberation and 
echo are heterodyned down to an audio fre- 


168 


CHARACTERISTICS OF ECHOES AND REVERBERATION 


quency, which has generally been in the neigh- 
borhood of 800 to 1,000 cycles. It is therefore 
relevant to inquire as to the possible effects 
which heterodyning may have on the back- 
ground. The spectrum will, of course, be largely 
unaffected, and the energy per cycle will have 
the same distribution about the new sonic mid- 



Figure 7. CRO trace for reverberation from 

271-millisecond pulse heterodyned to 800 cycles. 

frequency /o' as it originally possessed about 
the supersonic midfrequency /o. Similarly, the 
amplitude characteristics of reverberation 
would not be expected to be influenced appreci- 
ably by the heterodyning process. 

A CRO trace of heterodyned reverberation is 
reproduced in Figure 7. Comparison of such 
traces with those obtained for unheterodyned 
reverberation recorded at the same time shows 
that the abrupt reversals of phase found at 
various points in Figure 7 are much more com- 
mon in the heterodyned sounds. Theory indi- 
cates that this effect is especially likely when 
the amplitude is relatively low. Such phase 
changes may conceivably have an appreciable 
effect on the masking properties of the rever- 
beration. 


^ FM Reverberation 

When a pulse of rising or falling frequency 
IS sent out into the water, the returning rever- 
beration loses the marked tonality which it pos- 
sesses for CW pulses. Echoes from scatterers 
at different ranges have different frequencies 
at any one time, and the combination of these 
different echoes produces a sound which is 


similar to a wide-band noise. The spectrum of 
the reverberation will be similar to that of the 
pulse, although over a region many thousands 
of cycles wide, exact similarity is not to be 
expected. While the total intensity of FM rever- 
beration is the same as that of CW reverbera- 
tion, the energy per cycle will be less, since the 
same energy is spread over a much wider band 
of frequencies. The energy of the echo per cycle 
is similarly reduced, of course. 

Reverberation from an FM signal is also 
characterized by a different type of amplitude 
modulation than is found for CW reverbera- 
tion. In the first place, the variability from the 
mean intensity Iq at any time, as determined 
by measurements in successive reverberations, 
is less. For the Rayleigh distribution, applicable 
to CW reverberation, the rms deviation of 
reverberation amplitudes is 52 per cent of the 
mean amplitude. For FM reverberation, on the 
other hand, the observed rms deviation is only 
33 per cent of the mean reverberation ampli- 
tude. 

In addition, the rate of variation is much 
greater for FM reverberation than it is for CW 
reverberation. In the latter case, the typical 
reverberation blob has about the same duration 
as the pulse, provided that the pulse duration 
is much less than the elapsed time between the 
emission of the pulse and the arrival of the 
reverberation which is to be measured. Since 
the duration of the echo tends to be about the 
same as that of the pulse, except for very short 
pulses and extended targets, a blob of CW 
reverberation tends to have about the same 
shape as an echo. This naturally increases the 
difficulty of echo recognition. With an FM pulse 
the duration of the echo is unchanged but the 
length of a typical reverberation blob becomes 
very much less. This difference between FM 
reverberation and echo makes it possible to 
smooth out the reverberation without also 
smoothing out the echo; the echo thus stands 
out clearly above the smoothed reverberation. 
Similarly, the aural detection of an echo with- 
out doppler presented against a reverberation 
background, is improved somewhat by the use 
of FM. The precise extent of this improvement 
is uncertain, since no detailed measurements 
have been made. The loss of doppler discrimina- 



REVERBERATION 


169 


tion with FM pulses is such a serious disadvan- 
tage, however, that FM signals are primarily 
useful in such applications as mine detection. 
In this case, the small size of the target makes 
it possible to use very short signals to reduce 


reverberation, without reducing the strength of 
the echo. With such short pulses and with no 
doppler present, visual detection is probably 
preferable to aural in any case, and full ad- 
vantage may be taken of FM pulses. 



Chapter 8 

NOISE MASKING OF ECHOES 


M ost of the data described in this chapter 
relates to the recognition of artificial and 
of recorded echoes, from CW pulses, in the 
presence of noise (usually, thermal noise). As 
in Chapter 4, observations are grouped initially 
according to place of origin, because of differ- 
ences of procedure followed at various labora- 
tories, and summaries of general trends are 
given later in the discussion. 


underwater sound) which diminishes differ- 
ences between the time-amplitude patterns of 
the echo and the noise peaks should hinder 
detection. Conversely, distinctive modulations 
of the echo may be expected to aid detection. 
These, and similar points are discussed here 
in connection with the experimental evidence. 

^ ^ ^ General Procedure 


« 1 BELL TELEPHONE LABORATORIES 
TESTS 

Studies have been made of a number of fac- 
tors which may affect the practical situation, 
such as pulse duration, heterodyne frequency, 
system band width, and listening level, as well 
as various modulations, distortions, and time 
patterns. Most of the general ideas developed 
during the course of the previous analysis 
apply also to the masking of pulses. In Chapter 
2, for example, the width and, indeed, the con- 
cept of a critical band have been defined for 
relatively sustained tones only. These definitions 
seem to require no substantial modification 
even for fairly short pulses; in other words, 
differences in pitch of echo and background 
components provide a helpful cue. On the other 
hand, recognition of tonal pulses would be ex- 
pected to deteriorate with diminishing pulse 
length since the loudness of pulses is a function 
of their duration (see Figures 12, 13, and 14 
in Chapter 2), and loudness, or degree of 
neural stimulation, is related to masking. In 
addition, chance noise peaks which coincide 
with, or slightly precede the echo cut its effec- 
tive duration ; hence, the detectability of pulses 
should continue to improve even when their 
lengths exceed 250 milliseconds (see Figure 8). 
For this reason, the “squareness’' of the echo 
envelope (the rates of growth and decay at the 
leading and trailing edges) should help the 
listener, and any process (such as constrictive 
filtering or envelope distortion produced by 
phase interference during transmission of the 


In these tests,^ the masking background was 
a band of thermal noise extending from 200 to 
3,000 cycles and essentially fiat over that inter- 
val, which is a fair approximation to the prac- 
tical situation (see Figures 66 through 75 in 
Chapter 4) . Fluctuations from the mean level of 
the noise in the 2,800-cycle band, as measured 
with a thermocouple-ammeter arrangement, 
were occasionally as great as ±1.5 decibels, but 
on the average did not exceed ±0.5 decibel. 
The test apparatus was essentially that shown 
in Figure 1 in Chapter 4, except that a pulse 
generator was used to feed the signal channel. 
The response of the headphones was very 
nearly fiat within the limits of the 2,800-cycle 
noise band. Tests were administered to groups 
of between 5 and 12 observers seated in a quiet 
room. Gain in the background channel had the 
same value in all the tests, but the gain applied 
to the output of the mixture amplifier could be 
adjusted by the individual observers to give the 
level each considered optimal for performance 
and comfort. 

Each test consisted of 20 signals, and during 
the time required to complete any test the noise 
background was presented without interrup- 
tion, that is, without subdivision into an alter- 
nating series of listening periods and quiet in- 
tervals. The signal strength and thus the sig- 
nal-to-background ratio were held constant for 
each group of 20 signals, which were injected 
according to a random time pattern in which 
the average interval between successive signals 
was about 8.5 seconds, although the spacing be- 
tween two signals was occasionally as little as 


170 


RESTRICTEIW 


BELL TELEPHONE LABORATORIES TESTS 


171 


2 seconds or as much as 15 seconds. The ob- 
servers were instructed to press a telegraph 
key momentarily, as soon as they thought they 
heard a signal, in order to encourage a respon- 
sive rather than a cautious attitude. The voting 
key actuated a counter designed to register 
separately all errors of omission and com- 
mission. Failure to depress the key within 1 
second after the injection of a signal was auto- 
matically scored as an omission, while a later 
vote (made at any time before the next signal) 
was scored as an error of commission; thus, 
random guesses were about 7.5 times more 
likely to be registered as commissive errors. 
The arrangement of the counting circuit pre- 
cluded the possibility that a long vote would 
register in more than one category. The fact 
that the results obtained in these tests are in 
good agreement with data obtained in tests in- 
volving a quite different scoring technique (see 
Sections 8.2 and 8.4) shows that a listener can 
decide and indicate within 1 second whether 
or not a signal is audible, at least, when the 
probability of perception is 50 per cent or 
better and when no distinction need be made 
between one kind of signal and another (such 
as wake echoes and submarine echoes). 

A series of tests was made for each type of 
pulse, the highest signal-to-noise ratio being 
used first in the series ; a rest period was given 
after each test to minimize fatigue. The signal- 
to-noise ratios used in successive tests were 
diminished in steps of 2 to 3 decibels so that the 
range between approximately 100 per cent and 
approximately 50 per cent recognition proba- 
bility was covered in a series of 4 tests. In 
order to give some weight to errors of com- 
mission, recognition probability was defined as 
(N—E-)/(N + E+)y where N is the number 
of signals presented, and E- and E+ indicate 
the number of errors of omission and com- 
mission, respectively, so that occurrence of a 
substantial number of commissive errors would 
significantly reduce the calculated value of rec- 
ognition probability. Since errors of commis- 
sion were relatively infrequent (see Figure 2), 
values computed from the preceding expression 
were very nearly equal to those which would 
be obtained by the procedure described in Sec- 
tion 8.2, that is, by rejecting all tests in which 
E+ is significantly greater than zero and de- 


fining recognition probability by means of the 
fraction (N—E-) /N. The transition curves 
obtained for the various kinds of signal and 
background used in these tests are discussed in 
the following section. 

8 12 Effects of Pulse Length and 
Band Width 

The intrinsic frequency of the CW pulses 
which were studied was about 740 cycles; their 
durations, 600, 200, and 67 milliseconds. Three 
filters (with nominal band widths of 15, 5, and 
1% cycles, respectively) were used to restrict 
the frequency range of the interfering back- 
ground. It will be observed that the band 
widths selected are very nearly the smallest 
required (twice the reciprocal of the pulse 
length in seconds; see equation (5) in Chapter 
7) to transmit rectangular pulses of the in- 
dicated durations without seriously reducing 
their energy. Since, in practice, pulses and in- 
terfering waterborne backgrounds are neces- 
sarily transmitted to the ear over the same cir- 
cuit, it is important to pass signal and noise 
through the same filter in order to avoid in- 
troduction of spurious effects in this type of 
test; see Section 9.1.2. This procedure was fol- 
lowed where necessary. The output envelopes 
of the originally nearly rectangular pulses were 
examined with the aid of a persistent cathode- 
ray oscilloscope [CRO] screen, and the ob- 
served shapes are reproduced in Figure 1. As 
indicated in Section 4.2.3, noise peaks are pro- 
longed by the action of constrictive filters so 
that the minimum duration of each peak ap- 
proaches twice the reciprocal of the filter 
width. 

The transition curves in Figure 1 give the 
observed recognition probabilities for several 
combinations of pulse length and band width. 
The abscissas refer to the ratios of the overall 
levels measured in the signal and noise chan- 
nels separately. Since all results are referred 
to a common basis (level of the 2,800-cycle 
noise band) , the amount of improvement pro- 
duced by a given change in the test conditions 
may be obtained by inspection. Since no ob- 
servations were made at signal-to-noise ratios 
giving substantially less than 50 per cent rec- 
ognition probability, the abscissas have been 


RESTRICTED^ 


172 


NOISE MASKING OF ECHOES 


drawn at the latter value for the sake of con- 
venience in reading the curves. 

Several aspects of the data shown in Figure 
1 are worth examining. To begin with, it may 
be asked whether the width of a critical band, 


for the case of a tonal pulse masked by wide- 
band noise, is equal to the width measured for a 
sustained tone of the same intrinsic frequency 
as the pulse. Use of the direct approach, of the 
type discussed in Section 2.3, would imply that 


RECOGNITION PROBABILITY APPROXIMATE PULSE SHAPES AT OUTPUTS 

OF NARROW FILTERS 



0.2 SECOND PULSE 




TIME IN SECONDS 


0.067 SECOND PULSE 



Figure 1. Effects of pulse length and band width on t^ detectability of pulses masked by noise. 


BELL TELEPHONE LABORATORIES TESTS 


173 


this question should be answered in the nega- 
tive. Thus, the peak intensity of a pulse 
primaudible at 740 cycles, at which frequency 
the critical band is about 38 cycles wide, should 
be equal to the intensity of the noise in that 
same critical band, in other words, equal to 
38/2,800 of the uniformly distributed energy 
in the 2,800-cycle noise band. Hence, the over- 
all signal-to-overall noise ratio at primaudibility 
should be —18.9 decibels (10 log 38/2,800). 
The observed values of the primaudible pulse- 
to-noise ratios and the apparent loss of audi- 
bility in decibels (which is equivalent to an ap- 
parent widening of the critical bands) relative 
to the primaudible sustained tone-to-noise ratio, 
are presented in Table 1 for the 3 pulse lengths 


Table 1. Auaibilitj' loss relative to critical-band value. 


Pulse length in milliseconds 

600 

200 

67 

Observed pulse-to-noise ratio 




in db __ 

-16.6 

-13.3 

-9.3 

Loss relative to —18.9 db in db 

2.3 

5,6 

9.6 


studied (see Figure 1). The same degrees of 
loss relative to a sustained tone are represented 
graphically in Figure 8, discussed at length in 
Section 8.4.4, where the filled-in triangles refer 
to the primaudible signal-to-noise ratios for the 
three pulse lengths named here, and the level 
of the horizontal line indicates the signal-to- 
noise ratio required for a very long signal. 

Although the preceding analysis definitely 
establishes the degree of loss in audibility due 
to diminished signal length (and this loss is a 
significant quantity in practice), it does not 
reliably indicate whether the loss should be 
assigned to change in critical band width as a 
function of signal length, or to other factors. 
It has already been mentioned that two other 
relevant factors may be expected to infiuence 
the recognition of short pulses: the smaller 
stimulation produced by a short sound (which 
should, in the absence of a masking background, 
affect only those signals that are less than 250 
milliseconds long), and the smaller likelihood 
that a short signal will be presented during a 
favorable low intensity interval in the noise. 
The latter factor (which will be designated the 
sampling effect in the following discussion) ob- 
viously depends on the time pattern and peak 
factor of the noise and may perhaps be profita- 


bly studied by the methods discussed in Chap- 
ter 9 in connection with reverberation back- 
grounds. On the basis of Figure 8 it would 
seem that the effects of these two factors may 
become significant for an individual pulse when 
its duration is less than 1,000 milliseconds, at 
least for thermal noise backgrounds. 

It can be shown that any change of critical 
band width that may occur with changing sig- 
nal length does not contribute significantly to 
the observed loss of audibility. That this is 
substantially the case, even for pulse lengths 
as short as 40 milliseconds, is indicated partly 
by the discussion of Figure 8 given in Section 
8.4.4 and partly by the following examination 
of the data shown in Figure 1. 

Consider any one pulse length r, first, when 
the presented background extends over the full 
2,800-cycle band, and secondly, when the pre- 
sented background is transmitted by one of the 
narrow filters; then note the relative perform- 
ance in the two cases. This procedure tends to 
eliminate all effects of pulse length associated 
with the ear's build-up time as such, although, 
of course, the build-up time of the test ap- 
paratus and therefore the physical nature of 
the stimuli are modified by insertion of the 
filters (these residual effects are discussed 
later). Similarly, it tends to eliminate all in- 
fluence of pulse length associated with the 
sampling effect although, here again, changes 
of system band width complicate the situation 
by altering the envelopes of the pulses and the 
noise peaks (these effects are also discussed 
later) . 

It may be assumed that the difference in 
audibility observed with and without the nar- 
row filter is not due to effects of pulse length, 
since that is the same in both cases. If the 
masking principles discussed in Chapter 2 are 
valid here, then the improvement in perform- 
ance resulting from use of the filter is due to 
the fact that its admittance band B is narrower 
than the critical band, of width A/, centered at 
the same frequency (at 740 cycles. A/ equals 38 
cycles) . In other words, use of the narrow filter 
reduces, in the ratio B/Af, the transmitted in- 
tensity of the masking noise, that is, of those 
components in the noise background which are 
responsible for masking. This reduction of 
masking power should improve performance by 


4,]|gS^TRICTED 


'CTED^ 


174 


NOISE MASKING OF ECHOES 


10 log (^/A/) decibels. When B is less than A/, 
this quantity is negative; in other words, the 
signal-to-noise ratio needed for primaudibility 
is diminished by this number of decibels. Its 
absolute magnitude is entered in Table 2 as the 


ences between calculated and observed improve- 
ments (the relative success of the preceding 
analysis in predicting improvement) are listed, 
for reasons stated below, under the heading 
“total distortion loss’’. Two cases are distin- 


Table 2. Effects of band width and pulse length. 


T 

in seconds 

B 

in cycles 

Improvement in db 

Total distortion loss 
in db 

Loss due to 
pulse 
distortion 
in db 

Calculated 

Observed 

B'z = 1 

fiT = 3 

0.067 

15 

4.0 

0.7 

3.3 



0.200 

15 

4.0 

4.3 

___ 

-0.3 

3'6 

0.200 

5 

8.8 

2.7 

6.1 

___ 

3.5 

0.600 

5 

8.8 

6.2 



2.6 



0.600 

^ 3 

13.6 

6.2 

7.4 

--- 

--- 


“calculated improvement.” When B is greater 
than a/, the ear receives no assistance from the 
filter in discriminating between the frequency 
compositions of the pulse and the noise, and 
the preceding relation ceases to express any- 
thing significant for the process of auditory 
perception. Thus, when B is less than A/ but 
not less than 2/t, performance should be im- 
proved by the indicated amount through use of 
the narrow filters, since the intensity of those 
noise components which are responsible for 
masking decreases as B/^f, whereas the domi- 
nant components in the spectrum of the pulse 
are not attenuated. Before proceeding to exam- 
ine the experimental data in detail, it should 
be noted again that the present analysis seeks 
to make a denial rather than an assertion ; it is 
concerned with showing that the inferior audi- 
bility of tonal pulses as compared with continu- 
ous tones is not caused primarily by diminished 
pitch discrimination, that is, by a widening of 
the critical band with decreasing pulse length. 

The quantities B and t listed in Table 2, as 
well as the values of their products, refer to the 
data in Figure 1 ; in other words, the “observed 
improvements” represent the differences, for 
50 per cent recognition probabilities and the 
stated pulse lengths, between the overall signal- 
to-overall noise ratios required for primaudi- 
bility with and without the narrow filters. The 
method of finding the “calculated improve- 
ment” has already been explained. The differ- 


guished here, depending on whether Bt is less 
or greater than 2. 

It should be borne in mind, when examining 
the differences between calculated and observed 
improvements, or total distortion losses, that 
the values listed in Table 2 are much less relia- 
ble than the general trends, for which several 
factors are responsible. 

First, the observed improvements are them- 
selves computed by taking differences (hence, 
already affected by two experimental errors) 
between signal-to-noise ratios obtained in tests 
in which the number and identity of observers 
were not always the same and in which the 
sensation levels of the 50 per cent signals were 
quite different.*'" 

Secondly, in several cases, a short extra- 
polation has been made in order to find the 
signal-to-noise ratio corresponding to 50 per 
cent detection probability. 

Finally, the narrow filters used in these tests 
have a frequency response which is a typical 
resonance curve ; the band widths of the equiva- 


^ It was observed during these tests that the gain 
setting adopted by the listeners was determined pri- 
marily by the level of the presented background. It 
may be inferred, therefore, that the gain settings gen- 
erally corresponded to a loudness level of between 60 
and 70 phons, that is, to a higher critical-band sensa- 
tion level in the case of the filtered sound than in the 
unfiltered. As indicated in Section 9.2.2, this factor 
may influence the results, though probably by no more 
than 2 to 3 decibels. 


♦ 




BELL TELEPHONE LABORATORIES TESTS 


175 


lent rectangular filters assumed in deriving the 
computed improvements were obtained by find- 
ing the number of cycles included between 
points 6 decibels down from maximum re- 
sponse, which is only an approximation, though 
probably a fairly good one. Another approxi- 
mation which is frequently used involves the 
— 3-decibel points. Measurements made during 
these tests, of the fractional energy in the 
2,800-cycle band transmitted by each of the 
narrow filters, give somewhat better agreement 
with the —3-decibel definition. Which is the 
most significant procedure to use, from the 
standpoint of the auditory process, is not clear ; 
hence the values of the nominal band widths 
used in this section are the ones apparently 
preferred by the authors of the original report.^ 
The reason for their preference is not specified, 
and the choice may well have been based on 
nonauditory considerations exclusively. 

The trends, then, are as follows. When dis- 
tortion is small (5 t = 3) , the observed improve- 
ment is very nearly equal to the calculated, 
which confirms the assumption that critical 
band width is not substantially different for 
pulses and sustained tones. When distortion is 
significant (Br = 1) , the observed improvement 
is comparatively small. The difference between 
observed and calculated improvements is rela- 
tively large, in fact, a 5-cycle filter gives less 
observed improvement in the case of a 200- 
millisecond pulse than is obtained with a 15- 
cycle filter, and the magnitude of the disagree- 
ment between observed and calculated values 
increases as the band width of the filter is 
diminished, presumably because the resem- 
blance is thereby increased between both the 
pitch and time-amplitude pattern of the trans- 
mitted signal and the transmitted background. 
In other words, constrictive filtering is injuri- 
ous because it distorts the background as well 
as the signal (hence, the term “total distortion 
loss”) ; it may be, therefore, that the somewhat 
poorer agreement between observed and cal- 
culated improvements, for the 600-millisecond 
pulse, is partly due to the effect of the 5-cycle 
filter on the background. 

The last column of Table 2 represents an 
estimate of the deterioration which may be ex- 
pected from distortion of the pulse envelope 
alone. The estimate is in good agreement with 


the data for rounded pulses shown in Figure 8 
and is of interest quite apart from constrictive 
filters, since the envelopes of real echoes may 
be distorted during transmission or reflection. 
This segregation has been made by noting that 
the observed improvement for a 67-millisecond 
pulse and a 15-cycle filter is 3.6 decibels less 
than obtained for a 200-millisecond pulse and 
a 15-cycle filter. Since the filter width is identi- 
cal in both cases, its effect in distorting back- 
ground is the same. Hence the loss of 3.6 
decibels must be due to the fact that pulse dis- 
tortion is greater for the 67-millisecond pulse 
{Bt = 1) than for the 200-millisecond pulse 
(5 t = 3) , as indicated by the pulse shapes shown 
in Figure 1, and similarly for the pulses 200 
and 600 milliseconds long in the case of the 5- 
cycle filter. It should be noted that the observed 
improvements used in computing the last col- 
umn of Table 2 are themselves independent of 
such factors as build-up time of the ear and 
sampling, because of the way in which they 
were derived; hence, the major effect of pulse 
length reflected in the last column is distortion 
of the signal envelope. 

It follows from the preceding discussion that 
narrow filters of optimum width (probably 
Bt = 2) may improve the performance of echo- 
ranging gear in the field under certain con- 
ditions. Unfortunately, however, these con- 
ditions are not compatible with the general re- 
quirements. Thus, narrow filters help signifi- 
cantly (bearing in mind that an improvement 
of 12 decibels in the detection process will 
double the maximum echo range only in the 
absence of severe thermal gradients and at- 
tenuation) when the pulse length is 600 milli- 
seconds or more, and such long pulses would 
have the additional attraction of being more 
audible because of the diminished effects of 
sampling and auditory build-up time. Long 
pulses are not practical, however, because they 
would increase the search time, offer less secu- 
rity, increase the level of reverberation and 
would probably give somewhat less accurate 
auditory range determinations. Background 
admitted by filters less than some 200 cycles in 
width has an unpleasantly tonal sound, and, 
even with the standard 1- to 2-kilocycle band 
width of current echo-ranging gear, sonar 
operators complain of growing “ping happy” 


176 


NOISE MASKING OF ECHOES 


on long cruises. Furthermore, the use of filters 
less than about 400 cycles in width risks loss 
of doppler-shifted echoes (see Chapter 10) . 
When noise is limiting, the existence of doppler 
has relatively little effect on the audibility of 
echoes (see Figure 14, also equation (4) in 
Chapter 10) and means are available by which 
the amount of doppler in the presented echo 
can be reduced nearly to zero (see Chapter 10), 
but when reverberation is limiting, the exist- 
ence of doppler tends to offset the masking 
effect of background (see Section 10.2.2). Fur- 
thermore, doppler gives tactically valuable in- 
formation; hence, attempts have been made to 
increase the amount of the doppler shift in the 
presented sounds. Finally, Figure 1 indicates 
that the relative advantage afforded by the 
narrow filters at the 50 per cent level does not 
persist at the higher signal-to-noise ratios; in 
other words, the rate at which detection im- 
proves with increasing signal-to-noise ratio is 
greater for rectangular pulses, and n is re- 
duced,'" as a result of the distortion associated 
with the narrow filters, from 3.6 (without 
filters) to between 2.0 and 2.7 (with filters) . 

These values of n are comparable to values 
obtained in other tests with pulse signals which 
are described later, and, it will be noted, are 
much the same as the values obtained from 
tests with ship signals, which were discussed 
in Chapter 4. In other words, the relation be- 
tween perception probability and signal-to- 
noise ratio does not appear to be strongly de- 
pendent on the band width of the masking 
background (such as noise and reverberation), 
the method of test administration, sensation 
level in the optimal band, or duration and time 
pattern of the primaudible component. 

The method of administration and scoring 
used in the tests under discussion provides in- 
formation on the relative frequencies with 
which actual signals are missed and nonexis- 
tent signals reported under various conditions. 
The available data are plotted in Figure 2, 
where both types of errors are expressed as 


b For a definition of n, see Section 4.1.4. If the 
“spread” of the transition curve is defined as the signal 
increase in decibels between 20 and 80 per cent recog- 
nition probability, then n equals 12 divided by the 
spread. 


fractions of the total number of signals per 
test,® and where both were found to increase 
progressively with diminishing signal-to-noise 
ratio. In the case of the CW pulses, there is a 
clear tendency for E+/E. to be larger in the 
tests involving constrictive filters (i5T = l) than 
in the others. Since constrictive filtering has 
less immediate interest, the line in this figure 
has been drawn to follow the remaining points. 
It will be observed that the ratio of E+ to E., 



CW PULS 

lES 

1 

O Br 
A Bt 
O WIC 

1 

» 1 

•3 

)E- BAND 

o ^ 

( 

> O 

a 



2 



Figure 2. Kelative frequency of errors of omission 
and commission. 

which may be used to specify the tendency to 
give false reports, does not exceed 0.1 when the 
detection probability is better than 60 per cent 
(fewer than 40 per cent errors of omission), 
and that this is about equally true of undis- 
torted CW pulses and of modulated pulses. 
When the detection probability is less than 60 
per cent (more than 40 per cent errors of omis- 
sion), the relative number of false reports be- 
comes significantly greater for CW than for 


c The number of commissive errors per signal may 
be a function of the interval between signals and of 
the overall length of the testing period. In other words, 
it would be useful to know whether tests patterned 
after the field situation (few signals, long listening 
period) would increase the relative number of false 
reports, or failures to report, or both. Such a test, for 
the case of ship signals, is discussed in Section 4.2.6, 
but nothing similar has yet been described for pulse 
signals. 


RESTRICTED 



BELL TELEPHONE LABORATORIES TESTS 


177 


modulated pulses.^ When a telegraph key was 
substituted for the observers during one part 
of this test program, the automatic detector 
was found to give a larger E+/E. ratio than 
did the ear for the same test conditions. 

In organizing the data relating to masked 
thresholds of pulses, either of two standard 
reference bands may be used to define the prim- 
audible signal-to-noise ratio: (1) the 1,000- 
cycle reference band which is typical of many 
current echo-ranging installations (and this 
definition of the RD applies to the ordinates at 
the right of Figure 8), or (2) the somewhat 
more fundamental critical band, which may be 
taken as very nearly 50 cycles wide for all CW 
echoes heterodyned to frequencies between 0.1 
and 2.5 kilocycles (and this definition of the 
RD applies to the ordinates at the left of the 
same figure). When the reference band width 
is not specified in the ensuing discussion, the 
critical band RD is to be understood. This has 
the advantage that all recognition differentials 
are positive — or zero, in the case of a very 
long pulse or sustained tone. Furthermore, it 
focuses attention on those components in the 
background which are most directly responsible 
for masking. To convert from one reference 
band to the other, in the case of a background 
with uniform composition (which is usually 
true of the field, as well as the laboratory situa- 
tion) , note that the RD for a 1,000-cycle band 
is 13 decibels less than for a 50-cycle band (10 
log (50/1,000) = —13 db). Since critical band 
width varies somewhat with frequency, the use 
of the 50-cycle approximation between 0.1 and 
2.5 kilocycles gives results slightly different 
from what would be obtained with the correct 
critical band width, but such differences are 
usually less than 1 decibel. Thus, the values 
designated as the loss relative to a sustained 
tone, which are given in the first table of the 
present section, are the critical band recogni- 
tion differentials for rectangular CW pulses of 
the indicated durations. 

When the duration of such pulses is less than 

^ This observation does not necessarily mean that 
modulated pulses are to be preferred in practice, since, 
as shown later, they may be less detectable than CW 
pulses. The point here is merely that modulated pulses 
offer the listener an additional cue; he is, therefore, 
better able to classify some of the “doubtful” percep- 
tions. 


2/a/ seconds, where A/ is the critical band 
width, the essential spectrum of the pulse ex- 
tends beyond the confines of a single critical 
band. If the stimulation of two adjacent critical 
bands is no more effective than stimulation of 
either alone, it might be expected that the pro- 
gressive broadening of the signal spectrum 
with diminishing pulse length would produce a 
more rapid deterioration, in the audibility of 
CW pulses masked by wide band noise, than is 
observed for pulse lengths exceeding 2/a/ 
seconds. For / = 50 cycles, the change should 
set in at a pulse length of 40 milliseconds 
(2/50 = 0.04 second). 

Two observations discussed in Sections 8.4.2 
and 8.4.5 lend support to these remarks. First, 
the rate at which the audibility of 800-cycle 
CW pulses, less than about 50 milliseconds in 
duration, deteriorates is greater than that for 
longer pulses. Secondly, 6-kilocycle CW pulses 
with durations of 10 milliseconds or less are 
found to be only 1 to 2 decibels less audible 
than 1-kilocycle pulses of equal duration, al- 
though, on the basis of the critical band widths 
at these two frequencies, one would expect the 
1-kilocycle and 6-kilocycles pulses to differ in 
audibility by nearly 8 decibels [10 log 
(300/50) = 7.8 db]. The discrepancy of some 6 
decibels is of the expected magnitude, at least 
for a 10-millisecond pulse, as, in this case, the 
essential band is about 200 cycles (2/t) wide. 
Therefore, when the intrinsic frequency is 
about 1 kilocycle, the pulse energy is distributed 
among four contiguous critical bands, each 
about 50 cycles wide. Although the energy is 
unequally distributed among these four bands, 
equality of distribution may be assumed as a 
first approximation. If detection occurs when 
the signal-to-noise ratio assumes the same 
value, R, either in one of these four 50-cycle 
critical bands or in the single 300-cycle critical 
band centered near 6 kilocycles, then the 6-kilo- 
cycle pulse should be four times more audible 
than the 1-kilocycle pulse, and 10 log 4 = 6 db. 

There is also another set of observations, 
described in Section 2.2.2, in connection with 
the apparent loudness of pulses, which seems 
to have some bearing on the effects that occur 
when the essential spectrum of a tonal pulse 
extends beyond the limits of the critical band 
stimulated by the intrinsic frequency of the 



178 


NOISE MASKING OF ECHOES 


pulse. In that section, the discussion was con- 
cerned largely with the subjective loudness of 
the pulses; in the present section, the data of 



I 10 100 1000 


PULSE LENGTH IN MILLISECONDS 

Figure 3. Increments needed to maintain 125-cycle 

pulses at a loudness level of 60 phons. 

Figures 12, 13, and 14 in Chapter 2, are re- 
plotted in terms of the intensity levels (actual- 
ly, the sensation levels) of the sounds, in order 
to simplify comparison with the masking data 
shown in Figures 1 and 8. The replotted data 
are shown in Figures 3, 4, and 5, which were ob- 
tained by reading the points of intersection be- 
tween the curves in Chapter 2 and either of 
two horizontal guide lines (the 60-decibel line 
in one case, and the 20-decibel line in the 
other). For each of these two cases, the hori- 
zontal distance between the dotted line and the 
above-mentioned point of intersection repre- 
sents the number of decibels (called “the in- 



2 4601 2 4681 2 4681 


I 10 100 1000 

PULSE LENGTH IN MILLISECONDS 

Figure 4. Increments needed to maintain a 1-kilo- 
cycle pulse at the indicated loudness levels. 

crement” in what follows) by which the level 
of a pulse of the indicated duration must be in- 
creased in order for it to produce a subjective 
loudness equal to that of a particular sustained 
tone. Clearly, this sustained tone has a sensa- 


tion level of 20 decibels in one case and of 60 
decibels in the other. This procedure was car- 
ried through for each of the three frequencies 
shown in Figures 12, 13, and 14 in Chapter 2. 
In order that the increments so obtained repre- 
sent the number of decibels increase required 
for the tonal pulse in question, and not merely 
that of the equally loud 1-kilocycle tone, it is 
necessary to express the data of Figures 12, 13, 
and 14 in Chapter 2, in terms of sensation, 
rather than loudness level. These quantities are 
equal (by definition) at 1 kilocycle and essen- 



I 2 4681 2 4681 2 4681 

I 10 too 1000 

PULSE LENGTH IN MILLISECONDS 

Figure 5. Increments needed to maintain a 5.65- 

kilocycle pulse at the indicated loudness levels. 

tially equal at 5.65 kilocycles; they are quite 
different for a 125-cycle tone. The translation 
may be made by means of Figure 11 in Chap- 
ter 2. 

Owing to the paucity of the data and the 
curvature shown by the curves in Figures 12, 
13, and 14 in Chapter 2, in the neighborhood 
of 20 phons, the results for the lower loudness 
level are less reliable than those for the higher. 
For this same reason, the curve for 20 phons 
has not been included for the case of 125 cycles 
in Figure 3. 

It will be clear from the preceding descrip- 
tion that the increments shown in Figures 3, 4, 
and 5 represent a loss in loudness due to audi- 
tory build-up time. They are plotted in such a 
way as to be readily comparable with the loss 
of detectability (due to all causes and shown 
in Figures 8 and 14) which is suffered by a 
pulse, in the presence of masking noise, when 
its duration is reduced. In other words, the in- 


kESTRlCTED^ 


BELL TELEPHONE LABORATORIES TESTS 


179 


dicated increments are the amounts by which 
the intensity levels of pulses with various dura- 
tions must be augmented so that each pulse will 
produce the same loudness (and hence, the 
same stimulation) as a sustained tone of speci- 
fied loudness. 

Examination of the increment graphs shows 
that the points can best be fitted by a single 
straight line for the 125-cycle tone, but that 
two straight lines give a better fit for the 1- 
and 5.65-kilocycle tones. Where two lines are 
needed, they intersect at a pulse length very 
nearly equal to 2/a/. Since A/ is 260 cycles at 
5.65 kilocycles, and 50 cycles at 1 kilocycle, the 
discontinuities should occur at pulse lengths of 
8 and 40 milliseconds, respectively (2/260 and 
2/50) ; the observed breaks come at 10 and 40 
milliseconds. Adopting the criterion that the 
break should occur at a pulse length of 2/a/ 
seconds is equivalent to assuming that (1) any 
segment of the basilar membrane, with dimen- 
sions corresponding to a critical band, consti- 
tutes a resonance element of band width A/; 
and (2) the response of such a resonance ele- 
ment suffers when A/r is less than 2. Thus, 
when T is diminished, since A/ is approximately 
independent of r, the maximum in the basilar 
vibration pattern ceases to be as well localized 
as for longer tones, and the pitch and loudness 
functions show a more or less abrupt change; 
in other words, progressively larger numbers 
of its “channels” begin to function cooperative- 
ly, and the basilar membrane tends to as- 
sume some of the characteristics of a broadly 
tuned system. The 125-cycle graph shows no 
discontinuity, presumably because the low 
sonic frequencies already stimulate practically 
the entire basilar membrane, even at low sen- 
sation levels; in other words, pitch perception 
in this region differs from that at higher fre- 
quencies. A precautionary note should be added 
at this point: the increment graphs are based 
on a relatively small amount of data; they 
provide an indication rather than a proof. 
Moreover Figures 3, 4, and 5 have been ob- 
tained from the smooth curves drawn in Fig- 
ures 12, 13, and 14 in Chapter 2; the scatter 
of the observed points in the latter three fig- 
ures leaves the conclusions of the present dis- 
cussion open to some doubt. These tentative 
conclusions are, however, in agreement with 


the trend of the recognition differentials for 
short pulses discussed in Section 8.4.4. 

In connection with the response time of the 
ear, it should be noted that some sense of pitch 
remains even for relatively short stimuli. Thus, 
the value of the ear’s response time depends, 
to some extent, on the task set and the criterion 
used to rate its performance. For example, 
loudness begins to deteriorate at about 250 
milliseconds, presumably due to summation 
effects in the acoustic nerve. The pitch function 
seems to show a discontinuity at 2/a/ seconds, 
but pitch perception is possible (in the middle 
range of frequencies) down to 10 milliseconds 
for rectangular pulses and to 3 milliseconds for 
rounded pulses, and these limits seem to be set 
by the response time of the overall mechanical 
structure of the ear. Finally, judgments of 
phase differences at the two ears permit sub- 
division of the 1-millisecond interval between 
successive impulses conducted by a nerve fiber, 
so that sounds may be localized even when the 
difference in their arrival times is of the order 
of 10 microseconds. Each of these durations is 
significant for a specific problem, but no one of 
them is the response time of the ear. 

Sensation levels of 20 and 60 decibels have 
been adopted in constructing the increment 
graphs because these values are probably fairly 
close to the upper and lower limits encountered 
in standard echo ranging practice, and when 
receiver gain is set for a comfortable listening 
level. The slopes of the increment graphs 
depend on sensation level, the rate of deteriora- 
tion with falling pulse length being greater at 
the higher sensation levels, although the inter- 
cepts are in all cases of the order of 250 milli- 
seconds. It cannot be assumed, however, that 
better recognition differentials will be obtained 
when the gain is set for a low listening level, 
since obviously several factors besides auditory 
build-up time combine to give the observed de- 
pendence of primaudibility on pulse length. 
The fact, alone, that recognition differentials 
begin to deteriorate when the signal is reduced 
below about 1,000, instead of 250 milliseconds, 
is sufficient indication that this is so. Further- 
more, the transient click which contributes to 
the broadened spectrum of a short pulse, and 
which is associated with loss of tonality, and 
also with the augmented loss of loudness for 



180 


NOISE MASKING OF ECHOES 


pulses less than 2/a/ second in duration, is it- 
self a useful cue when a pulse must be detected 
in the presence of masking noise.® Assistance 
from this factor will, of course, be greater for 
the longer pulses (in which the click is con- 
nected with and calls attention to a signal whose 
duration exceeds that of the random noise 
peaks) . When the signal length is very short, 
approaching the duration of the noise peaks, 
relative squareness of the signal envelope 
ceases to constitute a very significant cue. In 
the latter case, the relative intensities of signal 
“pops” and noise “pops” tends to be the factor 
of major importance, and the results obtained 
for pulses of different frequency and envelope 
tend to be much the same. 

Intensity alone, however, is not an altogether 
satisfactory basis for echo detection. Consider 
a 1-millisecond pulse; according to Figure 8, 
such a pulse becomes primaudible (in a short 
laboratory test, and when the observers know 
that signals are presented fairly frequently) 
when its level is about 12 decibels above the 
mean level of the noise in a 1,000-cycle presen- 
tation band. In the field situation random noise 
bursts, not readily distinguishable from 1-milli- 
second echoes on any ground but amplitude, are 
commonly encountered in the masking back- 
ground. This is due to the fact that pitch per- 
ception is practically nonexistent for 1-milli- 
second pulses; in fact, if the intrinsic fre- 
quency of the heterodyned pulse is below 1 
kilocycle, less than one cycle will be completed. 
In addition, the duration of noise peaks trans- 
mitted through a band width of 1,000 cycles is, 
like that of the signal pulses, about 2 millisec- 
onds {2/B = 2/1,000 = 2 milliseconds) ; in other 
words, for the system assumed above, noise 
peaks and signals will have durations of about 
2X10“^ second. Consequently, the average num- 
ber of noise peaks per second will be about 500, 
or [1/(2 X 10"^) ] . If the time-amplitude pattern 
of the actual background resembles that typify- 
ing the class of sounds grouped under the head 
of thermal noise, then the probability that a ran- 
dom peak will have 16 times (10 log 16 = 12 
db) the average intensity is or about 


® It is, in fact, by responding to the spectra of brief 
pulses, rather than to their envelopes as such, that the 
ear is able to distinguish between pulses of the same 
duration and intrinsic frequency but different shapes. 


10-^^ Hence, the total number of noise peaks 
per second with intensities equal to or greater 
than 16 times the mean noise level is 500 x 10■^ 
or 0.5 X 10"^ During a watch of one hour, 
therefore, at least one noise peak of this ampli- 
tude will occur. Notice, however, that peaks 10 
decibels above the average level will occur at a 
rate of 500 X = 500 X 10“^-^ = 0.025 per sec- 
ond, or better than 1 per minute. Thus, a lis- 
tener may fail to notice a 1-millisecond pulse 
when it is only 12 decibels above the noise in a 
1-kilocycle band, and when he is not expecting 
such a pulse. This fact may have important ap- 
plications in the use of submarine echo-rang- 
ing gear and acoustic fathometers, which are 
often required to yield a detectable echo with 
only a few pulses. It will also be difficult for 
the operator of antisubmarine echo-ranging 
gear to be sure that he has detected a single 
very short echo. Sending a sequence of 2 or 3 
short pulses and giving them a “signature” or 
rhythm which is not likely to characterize suc- 
cessive noise peaks may aid positive identifica- 
tion. Some observations were made in a series 
of field tests regarding the effect of signature^ 
in which two surface vessels equipped with 
echo-ranging gear participated. The operator 
on the target vessel often failed to distinguish 
between the random pops in the water noise 
and short pulses arriving from the other ves- 
sel, even when such pulses were fairly intense. 
Similarly, the operator on the echo ranging 
vessel was not always able to classify these 
short echoes with certainty although he had the 
advantage of knowing whether or not a pulse 
had been put into the water. Overheard signals, 
as well as returned echoes could be very much 
more readily classified under given noise condi- 
tions when a pair of pulses separated by be- 
tween 50 and 100 milliseconds were used in- 
stead of a single pulse. Such a pair of pulses 
was recognized as a distinctive “dit-dit.” 

In general, the slopes of the increment curves 
in Figures 3, 4, and 5 are in the neighborhood 
of 15 decibels per decade ; that is, the intensity 
for constant sensation level, in the absence of 
masking, is roughly inversely proportional to 
the 3/2 power of the pulse length. This rela- 

f This follows from the fact that the distribution of 
amplitudes for random noise obeys the Rayleigh law 
(see Chapter 7 and reference 2 in this Chapter). 


^IrestrictEd' I 


BELL TELEPHONE LABORATORIES TESTS 


181 


tionship is a convenient way to summarize the 
data; it cannot at the present time be consid- 
ered to have any fundamental significance, 
since the precise meaning of sensation level in 
terms of auditory mechanics is not known. In 
fact, the available information indicates that 
the magnitude of the smallest stimulus which 
will produce discharge of a nerve fiber is an 
exponential function of stimulus duration. It is 
not certain to what extent this behavior de- 
pends on general fatigue, nor how much it 
varies among normal, unfatigued observers. 
But it seems likely that its operation would 
tend to increase the range of variability among 
recognition differentials obtained from a repre- 
sentative group of subjects listening to pulses, 
so that the variability among individuals would 
be greater than is usually observed when sus- 
tained signals are used. 

It is interesting to compare the slopes of the 
increment graphs with the rates at which in- 
tensity discrimination deteriorates for ampli- 
tude-modulated signals (see Figure 16 in Chap- 
ter 2). In this latter case, it will be recalled, 
the amplitude of tones was modulated sinusoid- 
ally at various modulation rates, and the in- 
tensity limen was observed to increase pro- 
gressively for modulation rates in excess of 
3 cycles per second. Detection of a loudness 
change under these conditions is equivalent to 
recognizing a signal masked by a fixed-level 
background (see the discussion of Figure 54 in 
Chapter 4), thereby differing in one fundamen- 
tal respect from detection in the presence of 
a fluctuating, thermal noise background. The 
signal used in the intensity discrimination tests 
may be regarded as consisting of a string of 
rounded pulses, each pulse being one modula- 
tion cycle in length. This suggests plotting the 
deterioration in loudness discrimination against 
the duration of the modulation cycle, for pur- 
poses of comparison with the increment and 
the RD graphs. This replot is shown in Figure 
6, and it will be observed from this figure that 
the rate at which intensity discrimination de- 
teriorates is about 10 decibels per decade. In 
addition, the intercept comes at about 300 
milliseconds, which agrees with the behavior 
of the increment graph but is in contrast to the 
1,000-millisecond intercept shown on the RD 
graphs. The difference between the positions of 


the intercepts for the RD and the intensity dis- 
crimination data is perhaps due in part to the 
fact that the effective background component 
in the intensity discrimination tests had a fixed 


(D 

O 



K PULSE LENGTH IN MILLISECONDS 

z 


Figure 6. Increments needed to maintain detecta- 
bility for strings of rounded pulses. 

level, whereas the masking background in the 
echo tests consisted of fluctuating thermal 
noise; this effect is discussed more fully in 
Section 8.5. 

From the data plotted in Figures 3, 4, 5, and 
6, it is evident that the intensity increment for 
constant loudness varies between 10 and 20 
decibels per decade of pulse length, the precise 
value depending on the experimental condi- 
tions. This relationship holds only for dura- 
tions less than 250 milliseconds. For longer 
durations, full subjective loudness occurs, 
determined only by the intensity level and fre- 
quency of the sound. For sounds of less than 
250 milliseconds duration, the ear’s response 
depends more upon the total energy in the pulse 
(power times duration) than it does on the 
power alone. 

It is sometimes assumed that for short pulses 
the response of the ear is directly proportional 
to the total energy. Figures 3 through 6 show 
that in some cases the response seems to vary 
more rapidly with pulse lengths than would be 
expected from this simple relationship. The 
available data indicate that, for constant loud- 
ness, the intensity of the pulse should be in- 
creased more nearly as the inverse % power of 
the pulse length than as the inverse first power. 
In view of the lack of accurate data, however, 
it is possible that the inverse first power is 
more nearly the correct relationship and that 
for short pulses the ear’s response is deter- 
mined primarily by the energy of the pulse. 


182 


NOISE MASKING OF ECHOES 


Various aspects of the auditory process have 
been reviewed and correlated in the foregoing 
pages in order to provide a certain amount of 
setting and perspective for the observations 
described in the remainder of this chapter. In 
general, recognition of CW pulses in the pres- 
ence of noise depends on the spectrum and 
time pattern of the background, the band width 
and center frequency of the signal spectrum, 
the duration of the pulse, and the integrating 
time of the ear. 

® ^ ^ Detection of FM and AM Signals 

Measured recognition differentials for each 
of three kinds of modulated signals masked by 
noise are shown by the open triangles in Fig- 
ure 8. The width of the noise band as well 
as other details of procedure were the same as 
used in the tests with CW signals. The open 
symbols have been entered in Figure 8 for the 
sake of comparison with the points for rectan- 
gular CW pulses which are indicated by means 
of filled-in symbols. The line has been drawn 
to fit these filled-in symbols only. 

The open triangle appearing in the graph at 
a pulse length of 200 milliseconds refers to a 
pair of 200-millisecond pulses (with an intrin- 
sic frequency of 740 cycles) which were sepa- 
rated by an interval of 200 milliseconds. This 
corresponds to a square-wave amplitude modu- 
lation of the CW signal. Comparison with the 
RD for a single 200-millisecond pulse (shown 
by the filled-in triangle) indicates that a single 
repetition helps by about 3 decibels under these 
conditions. If the critical band RD has positive 
values for pulses between 250 and 1,000 milli- 
seconds long solely because of the operation of 
the sampling effect, then the latter accounts 
for a loss in audibility of about 5 decibels for a 
250-millisecond pulse (compare Figures 4 and 
8). Since a single repetition offsets 3 decibels 
of this 5-decibel loss, it is clear that additional 
repetitions can provide little further improve- 
ment. This statement must be modified when 
the pulse repetition frequency exceeds 18 to 20 
per second, in which case additional factors 
may enter into the situation (see Section 8.5). 
For such high repetition rates, however, the 
duration of the individual pulses must be dimin- 
ished if the signal is to consist of a string of 


pulses rather than a sustained tone. In one 
set of tests (see Figure 18), repetition of a 
1-millisecond pulse at a rate of 5.6 per second, 
continued for several seconds, appears to have 
helped by about 10 decibels with respect to the 
RD for a single pulse. Unfortunately, not 
enough data are now available to indicate 
whether this result is reliable. 

It does not appear likely that the improve- 
ment observed when the double pulse was used 
arose from the integrating action of the audi- 
tory mechanism, because the major effects of 
the first of the two pulses are not likely to 
have persisted more than about 0.14 second 
after its termination (see also Section 8.2). 
Nevertheless it is interesting to compare the 
audibility of this repeated 200-millisecond pulse 
with that of the 600-millisecond pulse, since 
both have equal overall duration. It will be 
seen from Figure 8 that performance obtained 
with the repeated signal is very nearly as good 
as that obtained with the 600-millisecond pulse. 
Use of the dit-dit signal would therefore give 
a slight advantage relative to the longer one 
when reverberation is limiting, because the 
strength of background would be reduced some- 
what by abbreviating the transmission. In ad- 
dition, it is probable that losses of echo inten- 
sity due to rapid changes in transmission con- 
ditions will not affect both members of such a 
pair of pulses equally, and in consequence the 
chance of detecting at least one of them would 
be improved. However, the use of a twin pulse 
is time consuming and is, in general, undesir- 
able during general search and screening oper- 
ations. 

The open triangle plotted at 400 milliseconds 
refers to a 400-millisecond signal consisting of 
two successive tones (0.8 and 1 kilocycle), each 
of 200 milliseconds duration. Since the ear can 
very nearly achieve maximum response during 
each of the 200-millisecond subintervals, the 
RD for such a signal would be expected to, 
and does, approximate the value for a CW 
signal with equal overall duration and with an 
intrinsic frequency of 740 cycles; in other 
words, critical band width is nearly equal for 
all three frequencies. 

The open triangle plotted at 110 milliseconds 
refers to a frequency-modulated pulse. This 
had a constant amplitude and its frequency 




RESTRICTEJl 


BELL TELEPHONE LABORATORIES TESTS 


183 


was increased at a uniform rate from a value 
of 400 cycles at its beginning to a value of 
2,500 cycles at its end. The recognition differ- 
entials for this pulse and for the pulse de- 
scribed in the preceding paragraph were com- 
puted from the ratio of signal intensity to in- 
tensity of noise in a l-kilocycle reference band ; 
that is, by adding 10 log 2,800/1,000, or 4.5 
decibels, to the observed RD for the 2,800-cycle 
presentation band. This procedure would be a 
useful basis for comparison even if the critical- 
band concept could not be invoked; but there 
is no reason to suppose that auditory discrimi- 
nation of pitch was involved in any fundamen- 
tally different way for the CW and for the two- 
frequency and multi-frequency pulses, as indi- 
cated later, and by the general consistency be- 
tween the recognition differentials for the three 
kinds of pulses. 

It will be observed that the FM pulse was 
about 2 decibels less audible than would be ex- 
pected for a CW pulse of equal length. Field 
observations also indicate"^ that, in the pres- 
ence of noise, an FM echo about 150 millisec- 
onds long is approximately as audible as a CW 
echo of equal amplitude and duration. It fol- 
lows that the RD plotted for the 110-millisec- 
ond FM pulse in Figure 8 is fairly reliable, and 
it is worth considering some of the factors 
which enter into this result, for the light it 
may shed on the auditory process involved, 
and also because FM pulses may have certain 
practical advantages when reverberation is 
limiting. 

As to the reliability of the laboratory tests 
with FM pulses, it should be noted that in the 
experiments described in the present section, 
the masking noise was always passed through 
the 2,800-cycle filter, and, in some tests, 
through the narrow filters as well. The pulses 
were always filtered when the narrow noise 
backgrounds were used, but not in other tests. 
When the spectrum of a pulse extends beyond 
the limits of the noise spectrum, and thus out- 
side the masked region, the results of listening 
tests may be misleading since, in the field, sig- 
nal and noise must reach the ear over the same 
system. This factor was of no consequence in 
the CW test series, since the essential spectra 
of all CW pulses were less than 2,800 cycles 
in width. Furthermore, the general agreement 


between field and laboratory observations in- 
dicates that no substantial error resulted from 
failure to filter the FM pulse. 

It is evident, first of all, that the results for 
FM pulses give very significant information on 
the nature of hearing. They strongly suggest 
that the build-up time of the ear is related to 
some post-cochlear stage of the auditory proc- 
ess. An FM pulse 110 milliseconds in length, 
sweeping over 2,100 cycles, takes only 3 milli- 
seconds to sweep over a critical band 35 cycles 
in width. Thus, if each critical band had its 
own build-up time, this FM pulse would be 
expected to have a recognition differential com- 
parable with that of a 3-millisecond CW pulse. 
Actually, the observed RD corresponds to a 
CW pulse of about 80 milliseconds. It may be 
inferred, therefore, that there is apparently an 
integrating center involved in the hearing 
process which can correlate events at widely 
separated points on the basilar membrane, 
summing these separate events into a single 
whole. 

It may be possible to test this conclusion by 
studying the loudness of FM pulses as a func- 
tion of overall duration and frequency sweep, 
although reliable loudness balances might be 
more difficult to make for FM than CW pulses. 

As to the detailed auditory process involved, 
it should be noted first that, in the field and 
in the presence of masking, FM echoes are 
heard as chirps or glides, provided the echo is 
not too brief or the frequency sweep too ex- 
tended. It seems reasonable, therefore, to con- 
sider the masking of FM pulses from the point 
of view applied to the problem of the sensed 
properties of glides as heard in the absence of 
masking. 

In the case of a glide, the physical stimulus 
may be considered^ as composed of a sequence 
of brief rounded pulses having slightly differ- 
ent frequencies and, of which, no more than 
two or three need be assumed to have substan- 
tial amplitudes simultaneously. Thus, the co- 
existing pulses combine to give a wave with 
constant amplitude and variable frequency. At 
the beginning and end of the glide, however, 
the component pulses have abrupt onsets or 
terminations, so that the FM wave must be 
considered as made up of large groups of fre- 
quencies at its termini, and these transients 


^ElEgTRICTK 


184 


NOISE MASKING OF ECHOES 


are heard as clicks without significant tonality. 
Between the clicks, the successive pulses stimu- 
late essentially the same segments of the basi- 
lar membrane as respond to sustained tones 
with the intrinsic frequency of the pulse. The 
locus of the maximum amplitude of basilar 
disturbance associated with successive pulses 
moves smoothly from one pitch region to the 
next because of the way in which the phases 
and amplitudes of the different pulses are re- 
lated. Furthermore, the sensed pitch moves 
up or down the scale in much the same man- 
ner as does the frequency of the objective stim- 
ulus. The relative absence of tonality at the 
ends of a rectangular FM pulse causes the per- 
ceived extent of the glide to be less than its 
actual extent; hence, it may be best to use 
rounded rather than rectangular FM pulses. 

In terms of the line-busy effect, the ability 
to perceive the various frequencies in an FM 
signal depends on whether or not the maximum 
amplitude of the engendered disturbance, 
which moves along the basilar membrane, ex- 
ceeds the effective amplitude of the noise tend- 
ing to jam reception in each of the critical 
bands successively stimulated by the glide. The 
results shown in Figure 8, and those obtained 
in the field, imply that (for a 100- to 150-milli- 
second pulse, with a sweep of 1 to 2 kilocycles) 
the effective amplitude at the maximum of the 
moving disturbance is very nearly of the same 
magnitude as that produced in the case of a 
CW pulse whose intrinsic frequency coincides 
with the center frequency of the glide. 

The relative audibilities of various FM 
pulses would be expected to depend on a num- 
ber of factors. Thus, the rate of sweep should 
probably not be too great. To make this state- 
ment somewhat more quantitative, consider 
the FM pulse used in the tests under discus- 
sion. This was swept through 2,100 cycles in 
0.110 second. The disturbance produced on the 
basilar membrane by such a stimulus probably 
has a fairly well localized maximum at any 
instant, but this maximum is very likely of 
appreciable amplitude over a segment corre- 
sponding to not less than the critical band 
width (about 50 cycles at the middle frequen- 
cies). Hence, a disturbance which sweeps over 
2,100 cycles in 110 milliseconds will produce 
nearly maximum stimulation of a short seg- 

cr- 


ment of the membrane for a period of a few 
milliseconds; and this duration begins to ap- 
proach the time threshold of pitch perception. 
This effect might be modified to some extent 
by increasing the listening level, which should 
increase the width of the maximum and there- 
by increase the length of time during which 
any segment of the membrane is stimulated 
by the traveling disturbance. 

Furthermore, the critical bands stimulated 
by a pulse which is swept from a frequency of 
400 cycles to one of 2,500 cycles are not equally 
wide, and their widths (or relative amounts 
of masking produced by backgrounds) at the 
lower and upper limits of sweep differ by a 
factor of nearly 3, or about 5 decibels. Thus, 
variations in critical band width alone seem 
sufficient to account for the 2-decibel difference 
in the audibilities of the approximately 800- 
cycle CW pulses and the FM pulse referred to 
in Figure 8. It is reasonable to infer that the 
recognition differential for an FM pulse against 
a noise background may be smallest (most fa- 
vorable) if the pulse is restricted to the fre- 
quency region in which the critical band width 
is less than 50 cycles; in other words, in the 
region between 100 and 1,500 cycles. 

When the duration of a rectangular FM 
pulse is reduced below about 30 milliseconds, 
the interval during which the ear senses the 
initial and terminal clicks becomes a very large 
fraction of the overall pulse duration. In ad- 
dition, if the sweep is large, the time which 
the signal takes to sweep over a critical band 
width becomes so short that pitch perception 
disappears. These two effects, together with 
the possibly related build-up time of the ear 
as a whole, may be expected to impair the 
audibility of FM pulses. 

Field tests indicate the extent of this impair- 
ment.^ It was observed that a 10-millisecond 
FM pulse was very much less audible in the 
presence of noise than a 10-millisecond CW 
pulse, unless the extent of sweep was small. 
The sweep should probably extend over at least 
one or two critical bands, however; otherwise, 
the glide becomes difficult to perceive, and the 
audibility of the glide itself may be a valu- 
able means of distinguishing between real 
echoes and misleading simulations. Thus, dur- 
ing the laboratory tests with FM signals, it 


Vi 


RESTRICTgl^ 


CUDWR-USRL TESTS 


185 


was ‘‘the almost unanimous opinion of the ob- 
servers that frequency modulated signals were 
more readily detected than single-frequency 
signals.” ^ The data taken during the same tests, 
however, show that the recognition differen- 
tials for FM pulses were actually less favor- 
able than those for CW pulses of equal dura- 
tion. In confirmation of this result. Figure 2 
implies, not that the FM signals were more 
audible than the CW, but that they were easier 
to distinguish from false cues when such sig- 
nals could be heard; that is, E+/E- is smaller 
for primaudible signals when they have a dis- 
tinctive signature. This may have a further 
advantage in practice by reducing uncertainty, 
tension, and fatigue. It should be noted, in 
this connection, that the slopes and shapes of 
the transition curves obtained with modulated 
signals were in all respects similar to those 
obtained in tests with CW pulses. 

By way of conclusion, it seems worth sug- 
gesting that further study may profitably be 
given to the masking of rectangular and 
rounded FM pulses by noise and reverberation 
backgrounds, and as a function of the modula- 
tion parameters: rate, range, direction, and 
type of sweep (such as linear and sinusoidal). 

82 CUDWR-USRL TESTS 

In these laboratory tests, the results of which 
have been informally communicated, masking 
was studied for rectangular CW pulses of 16 
milliseconds duration, and with intrinsic fre- 
quencies of 400, 800, and 1,600 cycles. One 
test was also conducted with a 1,000-millisec- 
ond pulse of 800 cycles frequency. The mask- 
ing thermal noise was 300 cycles wide and 
centered at the pulse frequency. In general the 
apparatus used was similar to that described 
in the preceding section. The test sounds were 
presented to groups of between 6 and 10 ob- 
servers by means of a high-quality loudspeaker 
and at a comfortable listening level. The test 
sequences consisted of groups of 30 listening 
intervals, each interval containing either a sig- 
nal or a blank. In any sequence of 30 presen- 
tations, the noise level was fixed, and the sig- 
nals were randomly and equally distributed be- 
tween 6 predetermined levels (not including 
the blank level) which covered a range of 12 


decibels in 2-decibel steps. This 12-decibel 
range was selected to include detection proba- 
bilities from nearly zero to nearly 100 per cent. 
The procedure described here (randomized 
pulse levels) differs from that discussed in the 
preceding section, since each test sequence in 
the former study featured only one signal-to- 
noise ratio. The fact that the results of the 
two types of test are so consistent (see Figure 
8) implies that this difference in procedure is 
not very important. Listening intervals were 
about 3 seconds long, and were separated by 
silent intervals of the same length, during 
which the listeners reported their judgments 
of signal audibility. Rest periods were taken 
between the test sequences. 

Changes of overall gain did not affect per- 
formance significantly, which indicates that, 
at the listening levels used, masking was pro- 
duced exclusively by the thermal noise back- 
ground. However, the level of room noise was 
very much lower than would be encountered 
on the bridge of an antisubmarine vessel, and 
it may be that the distracting and fatiguing 
effects of loud, environmental sounds are more 
important in the field than are the masking 
effects of those sounds. 

The observed, primaudible signal-to-noise ra- 
tios for the 300-cycle noise bands were con- 
verted to the equivalent critical-band RD (us- 
ing the average 50-cycle width) by adding 10 
log 300/50, which amounts to 7.8 decibels. The 
critical-band recognition differentials obtained 
in this way are entered in Figure 8 as filled-in 
squares. The justification for use of the criti- 
cal-band concept in the case of pulse signals 
has already been discussed. The average criti- 
cal band width of 50 cycles was adopted be- 
cause no significant differences in performance 
were observed for the three pulse frequencies 
used, although it should be added that the ob- 
servers found the lower frequencies pleasanter 
than the high, because the 300-cycle bands of 
noise were less shrill when they were centered 
at the lower frequencies and, in fact, the ob- 
servers felt that they were making finer audi- 
tory discriminations in the case of the 400- 
cycle pulse than for any of the others. Quali- 
tatively, however, the 16-millisecond pulses 
appeared to have little definite tonality, irre- 
spective of their intrinsic frequencies. 


186 


NOISE MASKING OF ECHOES 


In evaluating recognition differentials, scores 
were based exclusively on the performance of 
observers who were guilty of no more than one 
error of commission per test sequence (6 blank 
intervals being presented in each sequence). 
In all tests, the frequency of commissive er- 
rors in the blank intervals did not exceed 7 
per cent. From this, it may be inferred that 
the curve in Figure 2 does not continue to rise 
very steeply in the region where errors of 
omission exceed 60 per cent. 

In agreement with other observations, the 
slopes of the transition curves obtained in the 
16-millisecond tests give a value of n which 
lies between 2 and 3. For the 1,000-millisecond 
tests, the value of n was very nearly unity. 
However, the amount of data in the latter case 
is too small to warrant attaching any signifi- 
cance to this difference. Transition curves were 
obtained by finding the detection probability, 
at a given signal-to-noise ratio, for the entire 
group of observers. Transition curves were 
also drawn for the individual observers and 
a tabulation was made of the various signal- 
to-noise ratios at which each individual per- 
ceived 50 per cent of the signals in a given 
test sequence; then the average of these indi- 
vidual primaudibility ratios was computed. 
These two methods of calculating recognition 
differentials rarely gave results which differed 
by more than 1 decibel; such differences did 
not appear to vary in any systematic manner. 
Individual variability, however, is sometimes 
fairly large. Thus, it was noted in one of the 
tests that the entire transition curve for the 
least responsive observer was shifted by nearly 
4 decibels with respect to the curves given by 
the others; when a retest was conducted on 
the following day, his transition curve was only 
about 1.5 decibels below the average curve. 

The members of the test group agreed that 
the 1,000-millisecond pulse was not heard in 
a sustained fashion when the signal-to-noise 
ratio was insufficient to assure 100 per cent 
detection probability. Only short segments of 
tone, very much less than 1,000 milliseconds 
in length, could be sensed continuously during 
a single presentation, and apparently such seg- 
ments were heard during periods when the 
level of the masking noise momentarily fell 
below its average value. A similar indication 


of the existence of sampling effect was obtained 
in a few qualitative tests with a dit-dit signal 
consisting of two 2-millisecond pulses sepa- 
rated by about 1 second. These repeated pulses 
were much easier to detect than single pulses 
of equal amplitude and duration because, when 
one of the pair was blanketed by a nearly si- 
multaneous noise burst,® the other was usually 
clearly audible. The more audible of the two 
was just as often the first as the second mem- 
ber of the pair; in other words, there was no 
evidence that the ear integrated the energy of 
the two pulses. 

83 BRITISH TESTS 

The British have obtained useful informa- 
tion by means of the “echo injection” scheme. 
This consists of injecting a rectangular pulse 
at some point in the circuit of an echo-ranging 
receiver. If this receiver is mounted aboard 
ship, the type and level of masking noise are 
exactly what would be encountered in the field 
under the same conditions of operation. The 
signal may be injected either before or after 
the heterodyne stage and is presented to the 
ear at the usual output frequency, approxi- 
mately centered with respect to the frequency 
limits of the admitted, masking noise band. An 
audio frequency of 1 kilocycle was used for 
the pulse in the tests described in the present 
section, and the spectrum of the heterodyned 
noise was very nearly flat. 

In practice, echo-injection tests are con- 
ducted with the listening vessel either adrift or 
underway at various speeds ; the received noise 
being picked up at representative bearings of 
the receiving hydrophone. In most cases, mea- 
surements are made of the amplitude of the 
primaudible injected pulse, but not of the level 
of the masking noise. However, if the listening 
conditions remain constant (pulse length, gain 
and filter settings, heterodyne frequency), such 
measurements, of the variation with speed of 
the primaudible amplitudes of injected signals, 
give operationally useful information in that 
they reveal how much stronger primaudible 

g It is possible, of course, that the difference in audi- 
bility between two such pulses is related to the phase 
(see Figures 8 and 9 in Chapter 9) of the masking 
background as well as to its amplitude. 


iESTRlCTE 


BRITISH TESTS 


187 


target echoes must be at one speed of the 
searching vessel than at another. 

In contrast to this procedure, the data shown 
in Figure 7 were obtained^ by measuring both 
the noise and the primaudible signal levels at 
the input to the headphones worn by the ob- 



QOO 0.01 002 003 004 005 006 007 008 009 0.10 

NOISE OUTPUT IN RMS VOLTS 


Figure 7. Variation of primaudible signal level 
with the level of self -noise, at indicated speeds and 
relative bearings, for a fixed gain setting. Hetero- 
dyned noise was presented in a band which extended 
from 230 to 2,800 cycles. The signal was a 50-milli- 
second rectangular pulse, heterodyned to 1 kilo- 
cycle. The slope of the line, or the signal-to-noise 
amplitude ratio, equals 0.530. Therefore the signal- 
to-noise intensity ratio equals (0.530)2, or -5.5 
decibels [20 log 0.530]. The equivalent'ratio for a 
50-cycle noise band is greater by 17.1 decibels [10 
log (2,570/50)]. Hence the critical band recogni- 
tion differential equals 11.6 decibels. 

servers; consequently a critical band RD may 
be computed, and this has been entered as a 
lozenge in Figure 8. This value is in good 
agreement with the general trend of RD values 
shown in that figure. In fact, the same critical 
band RD was obtained for the 50-millisecond 
signal when the pulse-background mixture was 
passed through any of four filters whose audio 
admittance bands were 230 to 2,800 cycles (in- 
dicated in Figure 7), 500 to 1,400, 800 to 1,200, 
and 940 to 1,070 cycles respectively. In other 
words, the strength of the just detectable echo, 
for given conditions of self-noise, was not dim- 
inished by restricting the width of the masking 
noise band, because the components of the noise 
which masked the 1-kilocycle pulse were es- 
sentially those contained in the 50-cycle critical 
band centered at 1 kilocycle, and these noise 
components were transmitted equally well by 
all four of the experimental filters. 

The points in Figure 7 represent averages 
obtained from a large number of observations. 
This procedure was found necessary because of 


the fairly high degree of variability of received 
self-noise even for apparently identical operat- 
ing conditions. 

It will be seen that the amount of masking 
is directly proportional to the level of the mask- 
ing noise, and the masking vanishes when the 
noise vanishes. Furthermore, the linear rela- 
tion shown in this figure indicates that the 
masking efficiency of the noise was identical 
for each of the four combinations of speed and 
bearing specified; even when the noise has a 
prominent amplitude modulation (as in the 
case of the 180-degree orientations), the only 
feature of the masking background which is 
statistically significant is the mean value of 
the rms noise level. 

It may be inferred, therefore, that target 
noise, which often has a large degree of am- 
plitude modulation when the target is a sur- 
face vessel, will not have more masking effi- 
ciency than, for example, deep-sea ambient; 
field experience^ has shown that this is true, 
even for pulses as short as 10 milliseconds. It 
was not true, however, for nonauditory meth- 
ods of detection in which intensity is the major 
cue and in which masking of pulses becomes 
more effective when the noise has a high peak 
factor. 

It should be added that primaudible signal- 
to-noise ratios were determined, in these tests, 
by the method of minimal increments. Such 
primaudibility values usually exceed the 50 per 
cent recognition differentials obtained from 
transition curves by about 1 to 2 decibels (see 
pp. 188, 231), and it may be partly due to this 
fact that the lozenge at 50 milliseconds tends 
to lie a little above the line drawn through 
the other filled-in symbols. It is also worth 
noting that the report describing the tests 
under discussion states that the gain was set 
for a comfortable listening level and that the 
primaudibility ratio changes somewhat when 
the listening level is varied over a wide range. 
It is not known whether this variation was 
produced by the alteration of critical band 
width with sensation level, by the greater im- 
portance of environmental noise at low listen- 
ing levels, by distortions due to overloading, or 
by some other factor. 

It was also found that hydrophone effect 
(listening for propeller sounds received by the 


188 


NOISE MASKING OF ECHOES 


echo ranging transducer) obtained on the 
screws of a vessel running a course parallel 
to that on which the tests were performed 
began to suffer when the listening band width 
was reduced much below 800 cycles. In other 
words, when the band width is insufficient to 
admit the fundamental and prominent har- 
monics of the modulation wave form, the char- 
acter of the signal modulation is poorly de- 
fined and may become unrecognizable. 

Band widths greater than 800 cycles gave 
no advantage in listening either to screw 
sounds or to pulses. In fact, admission of fre- 
quencies above 1,400 cycles annoyed the lis- 
teners to some extent because the sound be- 
came harsher. As in most supersonic listen- 
ing, the presented spectrum was fiat and the 
sensation levels of the high-frequency compo- 
nents were relatively high; hence, the large 
degree of annoyance. 

8 4 UCDWR TESTS ON SINGLE PULSES 

Some of the descriptive material in the pres- 
ent section will be found in reference 6. The 
remainder of the data collected here have been 
privately communicated by UCDWR in ad- 
vance of publication, which is expected in the 
near future; for more complete information, 
the forthcoming report on this subject, to be 
issued by UCDWR, should be consulted. 

® ^ ^ General Procedure 

Recognition tests were conducted in the pres- 
ence of thermal or recorded self-noise with iso- 
lated pulses and with strings of pulses re- 
peated at various rates. The work with iso- 
lated pulses is discussed first. The general test 
procedure used in either case was the follow- 
ing. Mixtures of signal and background were 
presented to groups of 3 to 5 observers by 
means of high-quality headphones, in a fairly 
quiet room, and at a comfortable listening level. 
Gain applied to the signal-background mixture 
could be adjusted for maximum comfort by the 
individual observers. The signal-to-noise ratios 
covered the range from nearly zero to nearly 
100 per cent recognition probability in steps of 
2 decibels. In any test sequence, gain in the 
background channel was held fixed and the 


various signal-to-noise ratios were equally and 
randomly distributed among the successive lis- 
tening intervals. The observers expressed their 
judgments by means of a well-adjusted hand 
key which could be switched from a neutral 
position to either of two others corresponding 
to audible and inaudible respectively. Transi- 
tion curves were plotted for all observers whose 
responses showed no significant number of com- 
missive errors for the blank presentations, and 
the 50 per cent values so obtained were used to 
represent primaudible signal-to-noise ratios. 
Rest periods were taken between test sequences 
in order to minimize fatigue. 

None but CW signals were studied in these 
laboratory tests. Details of pulse length and 
band width are given below in connection with 
specific test results ; so also are statements re- 
garding deviations from the general procedure 
just described, for example, self-administered 
tests. 

In general, values of n for transition curves 
obtained in these tests fell between 2 and 3 
and did not appear to depend significantly on 
background type nor on whether the signal 
was an injected pulse or a recorded echo. The 
method of minimal increments gave recogni- 
tion differentials larger by 1 to 2 decibels than 
obtained with the standard test procedure. 
Comparisons made in a few cases between 
transition curves and recognition differentials, 
obtained for given signal-background mixtures 
by means of either high-quality loudspeakers 
or headphones, showed no significant advan- 
tage for either method of presentation under 
the rather favorable laboratory conditions used. 

® Rectangular Pulses 

A heterodyne frequency of 800 cycles is 
standard in current American echo-ranging 
gear; hence, all recognition differentials ob- 
tained with pulses of or near this frequency 
have been collected in Figure 8 and indicated 
by means of filled-in symbols. The UCDWR 
tests add several points to this graph, three 
of them (for rectangular pulses 1, 149, and 
4,000 milliseconds in duration) being shown 
as solid circles. A typical rectangular pulse 
used is shown in Figure 9. The masking back- 
ground in these cases was thermal noise whose 


ESTRICTED 


UCDWR TESTS ON SINGLE PULSES 


189 



Figure 8. Aural detection of rectangular 800-cycle 
pulses masked by wide-band thermal noise. 


spectrum was nearly flat and which extended 
from 0.1 to 10 kilocycles for the tests with 
the 1- and 149-millisecond pulses. In the case 
of the 4-second pulse, the masking noise was 
admitted by one of two band-pass Alters, trans- 
mitting either one octave (565 to 1,130 cycles 
per second) or two octaves (400 to 1,600 cycles 
per second) ; identical critical-band recognition 
differentials were obtained with both of these 
Alters. A critical band width of 38 cycles has 
been used for all calculations involving 800- 
cycle pulses. It will be noted that the essential 
spectrum of a 1-millisecond pulse extends over 
an interval of 2,000 cycles (2/0.001). Conse- 
quently, a wide-band background is required 
to mask all the components in the pulse spec- 
trum, and conversely, a wide-band system is 
required to pass a 1-millisecond pulse without 
substantial distortion. 

In tests described in Section 8.5, rectangular 
800-cycle CW pulses with durations between 1 
and 10 milliseconds were repeated at various 
rates and presented against thermal noise back- 
grounds extending from 0.1 to 10 kilocycles. 
When a pulse of given duration was repeated 
at a regular rate (5.6 times per second), the 
critical-band recognition differentials for the 
strings fell along the dotted line shown in 
Figure 8. This line is included in the figure 
for comparison with the slope of the line drawn 
through the filled-in symbols, since the latter 
are somewhat sparse in the region of short 


pulse lengths. It will be observed that these 
lines have nearly identical slopes and that the 
recognition differentials for repeated pulses 
3 , are about 6 decibels smaller than for single 
oS pulses. 

30 (7) 

Ss ® Rounded Pulses 

-n -n 
m “n 

1 1 The envelopes of these pulses (with dura- 

tions of 100 and 200 milliseconds, respectively) 
ii are shown in Figure 9. In the absence of mask- 

” ing, these pulses had a slightly less “fuzzy” 

sound than did the rectangular pulses; their 
objective spectra are narrower or, what 
amounts to essentially the same thing, they 



140-MILLISECOND RECTANGULAR PULSE 



200~MILLISEC0ND lOO-MILLI SECOND 

ROUNDED PULSE ROUNDED PULSE 



BEAM ECHO FROM BEAM ECHO FROM 
79 -MSEC PULSE 36- MSEC PULSE 


Figure 9. CFO traces of rectangular and rounded 
CW pulses, and recorded echoes. 

generate less click transient within the ear. 

The masking background used in these tests 
was thermal noise with a flat spectrum, admit- 
ted by a band-pass filter with cutoffs at 400 





190 


NOISE MASKING OF ECHOES 


and 1,600 cycles per second. Critical-band rec- 
ognition differentials for the rounded 100-milli- 
second and 200-millisecond pulses are shown 
as open circles in Figure 8. The vertical lines 
drawn through these circles represent the dif- 
ference in performance between the best and 
poorest observers. These indications of the de- 
gree of scatter (which are fairly typical for 
rectangular as well as rounded pulses) have 
been included for comparison with the scatter 
in performance usually observed with recorded 
echoes and described in Section 8.4.6. 

It will be noted from Figure 8 that the 
rounded 200-millisecond pulse was about 2 
decibels less audible than a rectangular pulse 
of the same duration. This is in agreement 
with the estimated loss due to rounding which 
has been discussed in connection with Table 2 
and implies that the sharp onset and termina- 
tion of a rectangular pulse (the click tran- 
sient) provides a useful cue in distinguishing 
signals from false indications. It is not known 
to what degree the pulses emitted by practical 
sonar projectors are rounded, but it hardly 
seems likely that the majority of echoes re- 
turned by underwater objects will have ideal 
rectangular envelopes even when the outgoing 
ping does have such a characteristic. It is con- 
sequently helpful to have the results shown in 
Figure 8, in order to form some conception 
of the manner and degree by which recogni- 
tion of injected rectangular pulses differs from 
that of echoes encountered under service con- 
ditions. 


Recognition Differentials for 
CW Pulses 

The available data on the relative audibilities 
of rectangular 800-cycle pulses of various dura- 
tions are all plotted in Figure 8. Among the 
filled-in symbols, the triangles refer to 740- 
cycle pulses and the lozenge to a 1-kilocycle 
pulse ; the others, to 800-cycle pulses. As men- 
tioned in Sections 8.2 and 8.5, variations in 
observed signal-to-noise ratios required for 
primaudibility are almost negligible for pulses 
of these frequencies. Hence, the results are 
grouped in Figure 8 and collectively designated 
as applying to 800-cycle pulses. 


Although the plotted data are drawn from 
many sources, and were obtained under some- 
what different conditions of test, they appear 
to define a single function. The filled-in sym- 
bols (all points other than those obtained with 
rounded or modulated pulses) have been fitted 
provisionally by means of three straight-line 
segments, by analogy with Figures 3, 4, and 
5, although possibly a curve, or curves, should 
be used. In any case, the data seem to require 
a function which is concave upward and which 
has a larger (negative) slope in the region of 
shorter pulse lengths. Independent evidence, 
already discussed in Sections 2.2.2 and 8.1.2, 
also implies that the auditory function relat- 
ing RD and r is not a single straight line in 
the region of pulse lengths below about 600 
milliseconds. 

The results presented in Figure 8 may be 
summarized as follows. For signals between 1 
and 10 seconds in length, and presumably also 
for greater signal lengths, the critical band RD 
is zero ; thus, a long sustained tone can be recog- 
nized 50 per cent of the time when its power is 
just equal to the noise power in the correspond- 
ing critical band. For a 1-kilocycle reference 
band the RD is —14.2 decibels for an 800-cycle 
signal (10 log 38/1,000) . For signals less than 
1 second in length the critical-band RD in- 
creases (becomes less favorable) with decreas- 
ing signal length, with an overall change of 30 
decibels as the signal length is diminished to 1 
millisecond. This change is primarily due, of 
course, to the finite build-up time of the ear. 
However the data seem definitely to suggest a 
change in slope in the neighborhood of 50 to 
100 milliseconds, with slopes of 12 decibels and 
8 decibels per tenfold increase of pulse length 
for the shorter and longer pulses. The data are 
not sufficiently accurate, however, to establish 
the reality of this break in slope. 

These results are somewhat different from 
those obtained in the study of the loudness of 
pulses. A comparison of Figure 8 with Figures 
3, 4, and 5 shows in fact two major differences. 
In the first place, the intensity of the prim- 
audible pulse, in the presence of noise back- 
ground, increases as the pulse length decreases 
below 1 second, while for constant loudness, in 
the absence of masking, the intensity remains 
constant, down to 250 milliseconds. Thus, if the 


lES I KICFEDi 


UCDWR TESTS ON SINGLE PULSES 


191 


primaudible 250-millisecond pulse is compared 
with the primaudible 1-second pulse, in the 
absence of masking, the shorter pulse will 
sound louder than the longer one. In the second 
place, the slope of the RD curve shown in Fig- 
ure 8 is less than the slopes shown in Figures 3, 
4, and 5. Thus, the intensity of the just audible 
pulse, in the presence of noise background, does 
not increase as rapidly with diminishing pulse 
length as does the intensity of the pulse of con- 
stant loudness. Or, in other words, the loudness 
of the primaudible pulse, heard in the absence 
of the masking background, apparently de- 
creases with decreasing pulse length. Since 
loudness and masking have usually been found 
to be directly related, these results are some- 
what surprising and a provisional explanation 
of the discrepancy is given below in terms of 
the sampling effect. 

The increase of RD found for pulse lengths 
between 0.25 and 1.0 second may be attributed 
at least in part to the variability of the mask- 
ing noise background. The longer the pulse, the 
greater the probability that the noise level will 
fall sufficiently far below the average to make 
possible the recognition of the pulse. This phe- 
nomenon, which has already been referred to as 
“sampling effect,” is also suggested by the re- 
sults reported in Section 8.2 in which it was 
noted by the observers that the primaudible 
1-second pulse could not be heard for its entire 
duration but only during relatively low inter- 
vals in the background noise. 



PULSE LENGTH IN MILUSECONOS 

Figure 10. Deterioration of audibility, partly due 
to the sampling effect. 

A rough estimate of the quantity of interest 
is shown in Figure 10, obtained by plotting the 
difference between the recognition differentials 
found from Figure 8 and the intensity incre- 
ments required for constant loudness shown in 
Figure 4. The latter applies to 1-kilocycle 


pulses, but the properties of 1-kilocycle pulses 
are probably very similar to those of 800-cycle 
pulses. The data corresponding to a sensation 
level of 20 phons have been taken from Figure 
4. If the 60-phon data were used, the curve 
would be the same for pulse lengths between 
0.25 and 1.0 second but would come to zero 
more rapidly for the shorter pulses. It seems 
clear that the sampling effect should explain 
some of the difference shown in Figure 10, al- 
though the actual importance of this effect is, 
of course, uncertain. 

The disagreement between the slopes for the 
loudness and the masking curves has no im- 
mediately obvious explanation. It has already 
been noted in Section 8.1.2 that the results 
shown in Figures 3, 4, and 5 are not too reli- 
able, since the basic data on which they are 
based show an appreciable scatter. Thus it is 
not inconceivable that an intensity change of 
10 decibels per tenfold change in signal length 
may possibly characterize both the pulse of 
constant loudness and the pulse which can just 
be heard above the background noise. 

One additional discrepancy may be noted be- 
tween the masking data and the constant loud- 
ness curves shown in Figures 3, 4, and 5. The 
data in Figure 8 were obtained with nine dif- 
ferent noise bands, ranging in width from 0.3 
to nearly 10 kilocycles. In all cases the loud- 
ness of the noise background was adjusted to 
a value of about 70 phons. Thus, the noise 
energy contained in a critical band centered at 
800 cycles varied from about 20 decibels above 
the audibility threshold to about 65 decibels 
above this threshold. Despite this large differ- 
ence in critical-band listening level, no sys- 
tematic change of RD with band width was 
noted. This is in marked contrast with the data 
shown for 1-kilocycle pulses in Figure 4, where 
a considerable dependence on loudness level was 
found. The data are, again, not sufficiently pre- 
cise to yield reliable results; nevertheless, the 
practical conclusion seems indicated that the 
recognition differential is independent of lis- 
tening level for practical values of that level. 

The preceding discussion may be summarized 
as follows. The masking data shown in Figure 
8 seem to show significant differences from the 
data for pulses of constant loudness. Some of 
these differences may perhaps be explained in 


192 


NOISE MASKING OF ECHOES 


terms of the so-called “sampling effect.” Others 
are less readily explained and may or may not 
be real. It is entirely possible that the response 
of the ear may be quite different for very short 
pulses in the presence of masking noise, and for 
the same pulses in the absence of background. 

An effect already briefly discussed in Section 
8.1.2 may be considered next, namely, the broad- 
ening of the pulse spectrum beyond the limits 
of a single critical band. For an 800-cycle tone, 
the critical band is 38 cycles wide, and for a 50- 
millisecond pulse the essential spectrum width 
is 2/0.05, or 40 cycles. Thus, for pulses of given 
intensity which are less than 50 milliseconds 
in length, the signal energy in each critical 
band decreases with decreasing pulse length. 
By way of illustration, consider a 4-millisecond 
pulse whose essential spectrum has a width of 
approximately 500 cycles (2/0.004), and which 
therefore extends from a frequency of about 
550 cycles to one of about 1,050 cycles (800zb 
250 cycles). Such a spectrum stimulates at 
least ten contiguous critical bands, since no 
critical band in this frequency region is more 
than 50 cycles wide. It follows that the com- 
puted signal-to-noise ratio in each critical band 
is on the average about one-tenth of that shown 
in the figure, that is, 10 decibels lower. Since 
the distribution of pulse energy over the essen- 
tial width of the spectrum is not uniform, the 
signal-to-noise ratio for critical bands near the 
center of the pulse spectrum will exceed this 
average value. From equation (3) of Chapter 
7 it may be shown that the sound power per 
cycle at the midfrequency is twice as great as 
the average sound power per cycle in the “es- 
sential spectrum” of width 2/r cycles. For 
present purposes, however, this complicating 
factor may be neglected, and the average sound 
power is assumed to be uniformly distributed 
over the essential spectrum. This assumption 
appears reasonable when there is some coopera- 
tion between the different critical bands stimu- 
lated by a short pulse, and will in any case not 
be in error by more than 3 decibels. On this 
basis, then, the corrected value of the critical 
band RD may be considered as applying to any 
of the critical bands stimulated by the pulse 
spectrum, since the masking noise was essen- 
tially flat in all the pulse tests ; in other words. 


signal and noise spectra had approximately the 
same shape. 

Thus, an estimate may be made of primaud- 
ible signal-to-noise ratios for individual critical 
bands when the pulse spectrum occupies sev- 
eral critical bands and parallels the noise spec- 
trum. It was shown above that this correction 
is of the order of 10 decibels for a 4-millisec- 
ond pulse. Similar corrections have been ap- 
plied in the case of pulses between 1 and 40 
milliseconds in length. The corrected results 



Figure 11. Critical band recognition differentials 
for 800-cycle pulses masked by wide-band thermal 
noise. 


are shown in Figure 11, where the dotted line 
is transcribed from Figure 8 and represents the 
critical-band recognition differentials computed 
without regard to the width of the pulse spec- 
trum. Clearly, the signal-to-noise ratio in a 
single critical band tends to remain nearly 
constant for pulses which are so short that 
their spectra stimulate more than a single criti- 
cal band, but this approximate constancy per 
critical band is equivalent to an actual loss in 
performance because of the progressive broad- 
ening of the pulse spectra with diminishing 
duration. This application of the critical-band 
picture must be regarded as tentative at the 
present time, especially in view of its approxi- 
mate nature. It is shown in Sections 8.5.3 and 
9.2.2, however, that this extension of the criti- 
cal-band concept agrees with other experimen- 
tal data. 

For practical purposes, it is most convenient 
to present the data in the form used in Figure 
8 and to express the recognition differentials 
in terms of the 1-kilocycle presentation band 


restrigt: 


UCDWR TESTS ON SINGLE PULSES 


193 


on which the right-hand ordinates are based. 
This procedure is followed in the remainder of 
the discussion, because the effects of changing 
such factors as heterodyne frequency or sys- 
tem band width are most easily assessed by re- 
ferring all primaudible signal-to-noise ratios to 
the same standard. Thus, the use of the 1-kilo- 
cycle reference band in plotting points for the 
FM pulse data shown in Figure 8 shows much 
more clearly than could otherwise be done 
whether this procedure might be expected to 
help in the field. 

8.4.5 Effect of Heterodyne Frequency 

Inasmuch as the heterodyne frequency used 
in practice can be selected arbitrarily within 
rather wide limits, it is desirable to evaluate 
the effect which changes in this frequency can 
produce on overall performance. Changes in 
audio frequency might be expected to produce 
important changes in the masking of signals 
and in the precision with which doppler shifts 
can be recognized and identified. In addition, 
operator fatigue may be expected to depend on 
the heterodyne frequency used. The data pre- 
sented in this section are too preliminary to 
give conclusive results but are believed to shed 
useful light on the effects to be expected. 

Recognition tests were conducted with rec- 
tangular pulses of three frequencies (0.4, 2, 
and 6 kilocycles) in the presence of a nearly flat 
thermal noise band extending from 0.1 to 10 
kilocycles. The pulse lengths used in this work 
varied from 1 to 147 milliseconds. The test pro- 
cedure followed was, in general, the same as 
that described in Section 8.4.1. The only sig- 
nificant deviation was that all the masking 
data for pulses of 400 cycles, and some of the 
data for 6-kilocycle pulses were obtained in a 
self-administered test by an experienced ob- 
server. All the masking data at 2 kilocycles 
and some of the data for 6 kilocycles were de- 
termined in the usual manner with a number 
of listeners. General observations at UCDWR 
indicate that the results obtained in the self- 
administered test are not likely to differ by 
more than 1 or 2 decibels from the recognition 
differentials found in the more usual way. The 
same conclusion is evidenced by a comparison 


of the two types of results obtained at 6 kilo- 
cycles and plotted in Figure 13. 



PULSE LENGTH IN MILLISECONDS 

Figure 12. Audibility of 400-cycle pulses masked 
by thermal noise. The thick line, a visual best fit of 
the observed points, represents recognition differen- 
tials for 400-cycle pulses in a self -administered test. 
The thin line, taken from Figure 8, gives recog- 
nition differentials for 800-cycle pulses. The upper 
dotted line represents recognition differentials for 
200-cycle pulses, repeated 5.6 times per second. 

In a series of self-administered tests, the 
recognition levels were found for rectangular 
400-cycle pulses of different lengths. Masking 
was produced by thermal noise whose spectrum 
extended from 0.1 to 10 kilocycles. The recog- 
nition differentials found, referred to a refer- 
ence band 1 kilocycle in width, are plotted in 
Figure 12. The upper solid line in this figure is 
the mean curve taken from Figure 8, showing 
the corresponding recognition differentials for 
rectangular pulses of 800-cycle sound. Since no 
data were obtained at 400 cycles for pulse 
lengths between 150 milliseconds and 1 second 
in length, the dotted line in this region is in- 
tended only to carry the eye across this gap. 
The true position of the curve in this region is, 
of course, unknown. 

Figure 12 apparently indicates that 400-cycle 
pulses become primaudible when they are 3 to 
4 decibels weaker, relative to the adjacent noise 
background, than 800-cycle pulses of the same 
length. Although the slope shown for the 400- 



194 


NOISE MASKING OF ECHOES 


cycle curve is probably reliable, there are two 
reasons for questioning the absolute advantage 
indicated for pulses of this lower frequency. 
In the first place, the recognition differentials 
for sustained tones of 400 and 800 cycles differ 
by only a fraction of a decibel. In the second 
place, several lines of evidence indicate that 
for very short pulses there is little difference 
in the detectability of signals of 400 and 800 
cycles. For example, the tests discussed in 
Section 8.2 showed that for pulses of 16 milli- 
seconds duration, the recognition differential 
was the same for pulses of 400, 800, and 1,600 
cycles. Similarly, in the tests on repeated pulses 
described in Section 8.5, no change of recogni- 
tion differential was observed for a change of 
frequency between 0.5 and 6 kilocycles and for 
pulse durations of less than 10 milliseconds. 

This similarity between the recognition dif- 
ferentials of very short pulses of different fre- 
quency is generally consistent with the fact 
that such short pulses sound much the same 
regardless of the frequency. The essential spec- 
trum of a pulse 1 millisecond in length is at 
least 2,000 cycles wide. Thus, it is to be ex- 
pected that changes of center frequency as 
great as 1,000 cycles would have little effect on 
RD. It is evident physically that when a pulse 
is roughly 1 cycle long, the frequency of the 
pulse ceases to have very much significance. 

These expectations are confirmed by experi- 
ments on the observed pitch of short pulses. 
These observations were made in connection 
with the tests on repeated pulses, described in 
Section 8.5. Tonal pulses of various lengths 
were compared with noise pulses of the same 
lengths. The noise pulses were obtained from 
recorded water noise passed through a filter 
centered at 1 kilocycle. The band width of this 
filter was adjusted to admit a number of cycles 
equal to the reciprocal of the pulse length in 
seconds. The various tone and noise pulses were 
presented to several listeners both with and 
without wide-band masking background. Even 
when no background was present the observers 
found it impossible to distinguish between a 
pulse of pure tone and a pulse of water noise 
when these had a duration of less than 1 milli- 
second. Tonal pulses 5 to 10 milliseconds long 
gave a slight but rather indefinite semblance of 
pitch. Variation of the frequency of the tonal 


pulse from 700 to 1,000 cycles did not change 
these results. Similar practical observations 
made during field tests^ gave essentially the 
same results. 

It may be inferred, therefore, that the points 
plotted in Figure 12 for very short pulses 
should agree with the curve for 800 cycles. 
Thus, the evidence strongly indicates that some 
systematic error affects all the points plotted 
in this figure for 400-cycle pulses. Possibly in- 
adequate weight was given in this work to 
variations with frequency in the spectrum level 
of the masking noise (see Section 8.5.2). A 
part of the discrepancy may have arisen from 
the fact that the 400-cycle tests were self- 
administered. 

The upper dotted line shown in Figure 12 is 
taken from Figure 19 and shows recognition 
differentials obtained with a 200-cycle sequence 
repeated 5.6 times per second. This line is in- 
serted here for subsequent reference in Sec- 
tion 8.5.3. 



Figure 13. Audibility of 2- and 6-kilocycle pulses 
masked by thermal noise. The curve, a visual best 
fit of the circles, represents recognition differen- 
tials for 6-kilocycle pulses; the other line, taken 
from Figure 8, gives recognition differentials for 
800-cycle pulses. The open circles are observations 
made in a self-administered test, while the filled-in 
circles were obtained in a test administered to three 
observers. The triangles indicate recognition differ- 
entials for 2-kilocycle pulses in a test given to three 
observers. 

Recognition differentials obtained with rec- 
tangular pulses of 2 and 6 kilocycles are plot- 
ted in Figure 13. The masking background was 


HlESTRiCTEJiJ 


UCDWR TESTS ON SINGLE PULSES 


195 


again wide-band thermal noise, and the recog- 
nition differentials are referred to a 1-kilocycle 
reference band as in Figure 8. The open circles 
represent the data obtained in a self-admin- 
istered test at 6 kilocycles, while the filled-in 
circles represent the corresponding data ob- 
tained with three observers in the usual man- 
ner. Evidently any systematic difference be- 
tween these two sets of points is less than 2 
decibels. One of the lines in Figure 13 again 
represents the recognition differentials found 
at 800 cycles, taken from Figure 8. The curve 
has been fitted to the 6-kilocycle points, and, as 
before, the dotted line has been sketched in to 
carry the eye across the region from 150 milli- 
seconds to a sustained tone of 1 second or more. 
No curve has been drawn for pulses of 2 kilo- 
cycles because of the paucity of the data. 

In Figure 13, as in Figure 12, the points for 
the shorter pulses lie systematically some 3 to 
4 decibels lower than the curve for 800 cycles. 
Since the data for 400 cycles, 2 kilocycles, and 
6 kilocycles were all obtained in the same test 
series, it seems likely that the recognition dif- 
ferentials for all three frequencies are affected 
by the same systematic error. It has already 
been pointed out above that the recognition 
differentials for pulses about 1 millisecond in 
length should be independent of frequency in 
the range between 0.4 and 6 kilocycles. If all 
the points shown in Figures 12 and 13 are 
shifted upward by 4 decibels, they will be in 
agreement with the 800-cycle data for the 
shortest pulses. This does not appear to be an 
excessively large shift in view of the many pos- 
sibilities of systematic error in psychoacoustic 
work. 

When the recognition differentials for pulses 
of 0.4, 2, and 6 kilocycles are increased in this 
manner, all the available observations seem to 
be in fairly good agreement. Figure 14 has 
been drawn to give estimated recognition dif- 
ferentials at 0.4, 0.8, 2, and 6 kilocycles on this 
basis. Although these curves can scarcely be 
regarded as experimentally established, they 
are probably the best that can be drawn on the 
basis of present evidence. The curves shown 
for the data at 2 and 6 kilocycles have been 
drawn smoothly. Abrupt changes in slope 
might be expected at 15 and 10 milliseconds 
where the essential pulse spectra become equal 


in width to the critical band widths at 2 and 6 
kilocycles, respectively. However, the evidence 
is insufficient to indicate such discontinuities in 
slope. 



Figure 14. Estimated recognition differentials for 
pulses of different frequencies, masked by wide- 
band thermal noise. 


It may be observed that the estimated curve 
for pulses of 6 kilocycles has a more gradual 
slope between 10 and 100 milliseconds than the 
curves for the other frequencies. This is in part 
due, of course, to the fact that for long pulses, 
the recognizable 6-kilocycle signal must be 
higher relative to the background than for 
lower-frequency signals of equal duration. It 
is possible that changes in the sampling effect 
with changing frequency may also be impor- 
tant in explaining the difference between the 
curves for different frequencies. Owing to the 
increased width of the critical band at 6 kilo- 
cycles, the total noise level in this band will 
vary more rapidly (see Figure 57 in Chapter 
4) . The effect of this variability on signal recog- 
nition will depend on the details of the hearing 
mechanism. If, for example, the perceived 
sound represents a time average of the sound 
received in a critical band, the variability of 
the noise level will be decreased as the critical 
band width increases. On this basis the sam- 
pling effect should decrease in importance with 
increasing critical band width. Definite conclu- 
sions on this subject, however, must await more 
extensive investigations of the performance of 
the ear. 


RE^TRICTED^ 


196 


NOISE MASKING OF ECHOES 


The preliminary curves shown in Figure 14 
appear to justify, from the standpoint of aural 
masking, the general choice of heterodyne fre- 
quency in current American and British sonar 
gear (0.8 and 1 kilocycle, respectively). Prob- 
ably any frequency between 400 cycles and 1 
kilocycle would provide the same recognition 
differential for echoes in the presence of a noise 
background. The overall justification of the hete- 
rodyne frequency used in practice must, how- 
ever, take many other factors into account. 
Among these other factors are the recognition 
differential for echoes in the presence of rever- 
beration, discussed in Chapters 9 and 10, and 
the ability to distinguish small doppler shifts 
of the echo relative to the reverberation. 

Recorded Echoes 

The echo recognition tests described in the 
previous sections used rectangular pulses arti- 
ficially produced. Since there are many differ- 
ences between such ideal pulses and the actual 
echoes obtained from submarines, a limited 
series of observations was also carried out 
with recorded echoes. The echoes used were 
obtained by echo ranging at sea on a submerged 
S-class submarine several hundred yards away. 
Film recordings were made of the received sig- 
nal, heterodyned to 800 cycles, and relatively 
free of background noise and reverberation. 
For the tests described here, the submarine was 
kept as close to beam aspect as possible. Under 
these conditions^ the returning echo is usually 
similar to the outgoing pulse, though rarely 
an exact reproduction. Thus, the results might 
be expected to be somewhat similar to those ob- 
tained with rectangular pulses. No tests have 
been made on the noise masking of the “smear” 
type of echoes usually obtained at bow, stern 
or quarter aspect. 

During the masking tests these recorded 
beam-aspect echoes were mixed electrically 
with thermal noise recorded on a separate film 
loop. The use of the recorded noise background 
gave a more nearly uniform noise level than 
can generally be obtained with a thermal noise 
generator. The mixture of recorded echo and 
thermal noise was passed through either a one- 


octave (565 to 1,130 cycles) or a two-octave 
(400 to 1,600 cycles) band-pass filter. The 
width of these filters seems to have had no 
significant effect on performance in the mask- 
ing tests. 

Test sequences were conducted using one in- 
dividual echo recording at each time. In each 
test the noise and echo film loops were run con- 
tinuously; the echo was thus injected into the 
noise at regular intervals of 3 to 5 seconds, 
depending on the length of the film loop. Since 
noise and echo loops were of different lengths, 
an echo was rarely repeated at precisely the 
same time in the noise loop. 

Noise level was measured by means of a 
thermocouple or a copper oxide type meter. 
In all cases, signal level was defined in terms 
of the peak amplitude of the echo. This was 
done by using an oscilloscope as a visual indi- 
cator for adjusting the amplitude of a sus- 
tained tone to the peak amplitude of the echo. 
The level of the sustained tone was then deter- 
mined by means of a meter. In a few measure- 
ments of longer echoes made by means of in- 
tegrating meters, it has been found that the 
average echo level is between 1.5 and 2 decibels 
below its peak level. 

In making each masking test, the signal-to- 
noise ratio was varied in a random fashion 
by means of an attenuator in the signal chan- 
nel. Blank intervals were occasionally pro- 
vided for evaluating the number of commissive 
errors. The scoring technique used was similar 
to that described in Section 8.4.1. Transition 
curves were plotted for groups of five observ- 
ers, and the recognition differentials are shown 
in Figure 15. These are composite points, 
showing the performance of the group rather 
than that of the individual observers. As 
pointed out in Section 9.2, the difference be- 
tween performance of the best and poorest 
members of the group may be as large as 7 
decibels when real echoes are used. The cir- 
cles in Figure 15 represent the average of all 
determinations made with individual echoes; 
since these are averages of decibels, they cor- 
respond to geometric rather than arithmetic 
means. 

In general. Figure 15, shows the same trend 
as shown in Figure 8 for rectangular pulses; 
thus there is a loss in audibility of somewhat 


h See Division 6, Volume 8, Chapter 23. 


I^STRICTE^ 


UCDWR TESTS ON SINGLE PULSES 


197 


less than 10 decibels for a tenfold reduction in 
pulse length. It will be noted that there are 
apparently significant differences between the 
recognition differentials for artificial pulses 
and for echoes. Such differences may be ex- 
pected as a result of the time-amplitude pat- 
tern of the observed echoes. 



LENGTH OF TRANSMITTED PULSE IN MILLISECONDS 

Figure 15. Aural detection of beam echoes masked 
by thermal noise ; echoes were recorded after 
heterodyning to 800 cycles. Circles represent mean 
recognition differentials for specific echoes, and ver- 
tical lines indicate average scatter among these 
mean recognition differentials. The curve, taken 
from Figure 8, gives recognition differentials for 
800-cycle pulses. 

In the first place, the average intensity of 
a real echo several hundred milliseconds in 
length is, in general, several decibels less than 
the peak intensity used in computing the RD. 
This would tend to give larger (less favorable) 
recognition differentials than for rectangular 
pulses whose peak and average intensities are 
equal. For shorter pulses, the difference be- 
tween peak and average intensity is apparently 
less, and this effect is presumably of dimin- 
ished importance. In the second place, real 
echoes tend to be somewhat prolonged com- 
pared with the outgoing pulse. This effect is 
less important for beam echoes than for those 
at other aspects. Even at beam aspect, how- 
ever, the returned echo may be 5 milliseconds 
or so longer than the outgoing pulse. For very 
short pulses this obviously leads to a consider- 
able prolongation of the echo. A longer echo 
is easier to hear than a shorter one. Since the 


echo length used in Figure 15 is actually the 
length of the emitted pulse, rather than the 
length of the observed echo, echo prolongation 
will therefore tend to give smaller (more fa- 
vorable) recognition differentials than the rec- 
tangular pulses of fixed length. 

Difference of tonality between real and arti- 
ficial echoes may also be responsible for dif- 
ferences in RD. The tonality of real echoes 
tends to be inferior to that of pulses. This 
effect is in part the result of amplitude varia- 
tions of the echo envelope but may also be due 
in part to abrupt changes of phase occurring 
during the echo. In general, echoes hetero- 
dyned to 800 cycles had definite tonality in 
these tests only when they were more than 
10 milliseconds in length. A shorter echo 
sounded like a crack or pop. Even fairly long 
echoes tended to sound “noiselike” when their 
envelopes were very uneven. Another difficulty 
encountered in the recognition of real echoes is 
the absence of the click associated with rectan- 
gular or nearly rectangular pulses. This type 
of cue seems to be more helpful in the case of 
the longer pulses, since the shorter have little 
tonality and are mostly click in any event. 

The plotted points shown in Figure 15 are 
consistent with these theoretical expectations. 
For example, two points are shown for 9-milli- 
second pulses. One of these was actually 18 
milliseconds long, as determined by measure- 
ments made on the film. The mean RD for 
this 18-millisecond echo, shown by the lower 
of the two points plotted at 9 milliseconds, is 
fairly close to the RD indicated for a rectan- 
gular 18-millisecond pulse. The other echo was 
observed to be about 9 milliseconds long, and 
its observed RD is much closer to that for a 
rectangular 9-millisecond pulse. Similarly, the 
general trend in Figure 15 shows a greater 
loss in audibility relative to the rectangular 
pulse data for the longer echoes than for the 
shorter. This would be expected from the in- 
creasing difference between peak and average 
intensity as pulse length increases, and also from 
the increasing loss of tonality relative to rec- 
tangular pulses. Figure 8 shows a similarly 
more gradual slope for rounded pulses as com- 
pared with rectangular ones. In view of the 
wide scatter of the data, however, this agree- 


198 


NOISE MASKING OF ECHOES 


ment with expectation must be regarded as 
suggestive rather than conclusive. 

The vertical lines through the circles in Fig- 
ure 15 show the average degree of variability 
in group recognition differentials which were 
obtained in successive tests with particular 
echoes. As indicated above, the total difference 
between the best and the worst performances 
by individual observers during an entire test 
sequence is even greater. Although these stud- 
ies were of a preliminary nature only, ten or 
more successive tests were made with some 
of the echoes (such as the 18-millisecond echo 
discussed previously) . No progressive changes 
in performance were found to occur from test 
to test. It may be, however, that further ex- 
perience would have reduced the variability 
to some extent, since constant practice seems 
required for the maintenance of peak perform- 
ance in listening tests with real echoes (see 
Section 9.2.2). This would be anticipated from 
the fact that real echoes are objectively more 
complex than rectangular CW pulses ; there are 
more cues which a skilled observer can rely 
upon and, hence, more variability among ob- 
servers. Compare, for example, the variability 
indicated in Figure 15 with that shown in Fig- 
ure 8. 

The shapes of transition curves obtained in 
this study with real echoes were for the most 
part about the same as observed in tests with 
rectangular CW pulses. The only major differ- 
ences observed were for the recognition of the 
9-millisecond echoes, in which case the slope 
of the transition curve was very much more 
gradual than for CW pulses of equal length. 
This slow rate of improvement with increasing 
signal level is probably related to the fac- 
tors producing general variability in echo rec- 
ognition differentials; cues are ill-defined, 
hence increases of intensity are not as helpful 
as with rectangular pulses. 

In conclusion, it should be emphasized that 
echoes received from bow and stern aspects 
of a target would in general be even more 
smeared than the beam-aspect echoes used in 
the tests under discussion. In addition, even 
beam echoes would be expected to give dif- 
ferent results when multiple transition paths 
can substantially prolong the observed echo. 
To obtain more general information on echo 


masking, studies are required with a wider 
variety of echo samples. In addition, it may 
be useful to obtain fundamental information 
on the effects produced by arbitrary variations 
in the envelope and phase of an artificial pulse. 

8.4.7 Effect of System Distortion 

Receiver output O may be related to input I 
in a very large number of ways. When O is 
directly proportional to /, the receiver is 
linear; in all other cases the receiver is non- 
linear. Although linearity is frequently an 
ideal with electroacoustical equipment, it is 
rarely achieved over more than a limited range. 
Since various kinds of nonlinearity may occur 
in practice, it is important to inquire to what 
extent nonlinearity may affect the recognition 
of target echoes. Several tests along this line 
have been carried out by UCDWR. 

Two important types of nonlinearity found 
in practice are represented in Figure 16. In 



Figure 16. Two types of nonlinearity. 


curve A, a change in /, when I is small, pro- 
duces very little change in 0; but with increas- 
ing 7, the output becomes more and more sensi- 
tive to small changes in the input. This situ- 
ation arises when I is proportional to 0^ as, 
for example, in doppler-doublers. In curve R, 
on the other hand, I and 0 are proportional 
if the input does not exceed the critical value 
7c shown on the curves. Within this range of 
input the receiver is practically linear. When 


RESTRICTED 


UCDWR TESTS ON SINGLE PULSES 


199 


I is increased above h, the output curve flat- 
tens off and approaches asymptotically a maxi- 
mum output 0,„. In this nonlinear region the 
receiver is said to be overloaded. Since the out- 
put will not exceed 0,„, no matter how great 
the input, the receiver is said to have a limiting 
action. Nonlinearity of the type shown in curve 
A is important in square law amplifiers. Non- 
linearity of the type shown in curve B is en- 
countered among practical types of gear which 
are designed to be linear over a specified oper- 
ating range. The UCDWR masking tests have 
been carried out only for receivers whose re- 
sponse is similar to that shown in curve B. 

When the input in an echo-ranging receiver 
increases beyond the critical input h and the 
receiver overloads, several effects may be pro- 
duced in the observed masking of echoes. This 
problem is particularly important when short 
tonal signals are to be detected in the presence 
of noise, since then the amplitude of the just 
audible signal often exceeds that of the back- 
ground. This difficulty may be overcome by 
diminishing the total gain, so that the system 
operates along the quasi-linear portion of the 
curve below Ic on the solid line in Figure 16. 
When the signal amplitude is equal to or less 
than that of the admitted noise band, as is 
usually the case for long tonal signals, the 
difficulties produced by overloading and limit- 
ing may be minimized by diminishing system 
band width, and thereby the total amplitude 
of the signal-background mixture. When the 
receiver has a small dynamic range it is often 
suggested that the operator set the gain so that 
background noise is barely audible. In this 
condition, the high level of reverberation re- 
ceived soon after transmission of a pulse will 
not overload the receiver for so long a time; 
thus, echoes can be heard at shorter ranges 
than would otherwise be possible. Similar re- 
marks apply to nonauditory detection. Finally, 
when the peak factor of the noise is very large 
compared with changes of amplitude produced 
by introducing a primaudible signal, that is, 
when the total “swing” is large, it may be 
helpful to use some limiting if this is done 
with caution. 

The fact that a limiter (or other nonlinear 
system) changes the envelope of the transmit- 
ted disturbance means that the composition of 


the output spectrum of the sound will be dif- 
ferent from its input spectrum. Thus, the ef- 
fects of distortion associated with limiting de- 
pend upon system band width, frequency re- 
sponse (gain at various frequencies), operat- 
ing point and extent of swing. It should be 
noted that such factors interact with each 
other; in other words, the effect of varying 
two of them jointly is not necessarily equal to 
the sum of the effects produced by independent 
variation. Consequently, it is probably best to 
study the effects of such changes by varying 
two or more factors from test to test and to 
examine the more important combinations of 
the various factors. 

In a small number of tests conducted at 
UCDWR, moderate and extreme degrees of lim- 
iting were studied for their effects on the 
primaudibility of echoes masked by noise and 
reverberation ; for the results of the latter 
tests, see Section 9.2.2. The primaudibility 
tests with noise backgrounds were made in the 
usual way, for given signal and noise pairs. 
Masking tests were made first without limiting 
and then with either of two degrees of limit- 
ing. The test apparatus, minus the distorting 
networks, showed no more than 2 to 3 per cent 
of distortion at any gain setting in the useful 
operating range. 

The first type of limiting involved only a 
gradual flattening of the tops of the positive 
half cycles of an oscillator tone transmitted by 
the apparatus, the amount of flattening being 
somewhat dependent on the signal level. The 
second type of limiter had a very marked rec- 
tifying action, allowing somewhat less than V 2 
cycle to be transmitted out of each full cycle 
of a sinusoidal input wave. 

Recorded 800-cycle beam-aspect echoes of 
about 100 milliseconds duration were used as 
signals, and wide-band thermal noise consti- 
tuted the masking background. The levels were 
measured at the input to the distorting net- 
work, in the manner already described for re- 
corded echo and noise samples (see Section 
8.4.6). They were also measured at the out- 
put. Recognition differentials computed on the 
basis of input and output levels were very 
nearly equal. The output values are reported 
in the present discussion. 


RESTRICTED 


7 


200 


NOISE MASKING OF ECHOES 


The signal-background mixture was passed 
through either of two filters which defined the 
listening band. The first of these transmitted 
components between 565 and 1,130 cycles; the 
second, components between 0.1 and 9 kilo- 
cycles. In some tests, the signal-background 
mixtures were presented to the observers 
through headphones, and in others by means 
of loudspeakers, both of which were of high 
quality. Both distorting networks changed the 
quality of the presented sounds. Signal and 
noise became “wheezy,” and more so for the 
severer condition of distortion. Observers ex- 
pressed annoyance after listening to distorted 
sounds. 

Preliminary results indicate that signal rec- 
ognition was not significantly impaired for the 
moderate degree of distortion; and this was 
true for either loudspeaker or headphone pre- 
sentation and for both band widths studied. 
When the extreme degree of distortion was 
used, with either phones or speaker, recogni- 
tion suffered by 1.0 decibel in the case of the 
565- to 1,130-cycle filter and by 2.6 decibels 
for the 0.1-9 kilocycle filter. These effects ex- 
ceeded the variability from all other sources 
and are regarded as statistically significant. 
The reasons for this variation with filter width 
are not clear, although changes in overall gain, 
in loudness level, and in the number of inter- 
modulation frequencies passed by the filter may 
all have been important. It seems clear, how- 
ever, that the types of distortion used in these 
tests (which probably exceed anything likely 
to occur in practice) do not seem to be seri- 
ously harmful. 

8 5 UCDWR TESTS ON REPEATED 
PULSES 

In the development of acoustic fathometers 
for use on submarines, it was suggested that 
the use of rapidly repeated pulses might offer 
some advantage. One of the objectives in 
fathometer design is to use a signal which an 
antisubmarine vessel would not be likely to 
detect, but which would, after reflection from 
the bottom, yield a recognizable signal at the 
submarine. To evaluate the possible useful- 
ness of repeated pulses in these and other ap- 
plications, a program of psychoacoustic tests 


was undertaken by UCDWR. The first factor 
of interest was the recognition differential for 
a string of pulses as a function of pulse length 
and pulse repetition frequency. Other factors 
which also affect the recognition differential 
and which were studied in this program, were 
the heterodyne frequency, the frequency com- 
position of the repeated pulses, the width of 
the listening band, and the time-amplitude pat- 
tern of the masking noise background. 

* ^ General Procedure 

To generate tonal pulses, a sustained oscil- 
lator tone was subdivided into segments of 
known duration by means of a gating circuit 
in the signal channel. With this method of 
pulse generation, a 1-millisecond pulse re- 
peated at the rate of 1,000 pulses per second 
would produce a sustained tone at the out- 
put; in other words, each 1-millisecond pulse 
would connect into the succeeding one without 
shift in phase and almost without transient. 

The pulses studied were 1, 2, 5, and 9 milli- 
seconds long. The 1-millisecond pulses were 
studied at a variety of regular pulse repetition 
frequencies (prf ) which could be adjusted from 
a rate of 4 to a rate of 600 pulses per second. 
Much of the work was done at a prf of 5.6 per 
second. For this prf, the interval between pulses 
is about 0.18 second and is therefore of sufficient 
duration to permit the ear to resolve the indi- 
vidual pulses. The effect of irregular pulse rep- 
etition was also examined. In this case the test 
administrator could control the occurrence but 
not the duration of the pulses by tapping a 
well-adjusted telegraph key. In this way, 
strings of irregularly spaced pulses could be 
produced, the spacing between successive pulses 
varying from an interval of about 1 second to 
intervals which were as short as the adminis- 
trator could make them. 

Signal level was defined in all cases as the 
peak level of the individual pulse or pulses 
used. When repeated pulses were investigated, 
each signal consisted of a string of pulses ex- 
tending over an interval of 3 to 5 seconds, 
during which the pulse level was held fixed. 

To study the effect of heterodyne frequency, 
a number of oscillator tones, varying in fre- 
quency between 200 cycles and 6,000 cycles. 


f RESTRICTED 


UCDWR TESTS ON REPEATED PULSES 


201 


were fed into the pulser. In all cases in which 
a pulse length of 1 millisecond and an oscillator 
frequency less than 1 kilocycle was used, the 
pulsed signals consisted of less than one com- 
plete cycle. This factor, however, appears to 
have had little if any effect upon pulse detect- 
ability. In addition to tonal pulses, pulsed 
thermal noise was studied. The means em- 
ployed to produce thermal noise pulses are de- 
scribed later in this section. 

All the masking background noises used in 
these tests were recorded on film. These back- 
grounds were obtained from four separate 
sources. Three of the recorded noises had power 
level spectra given by curve A in Figure 17 ; 



I 2 4681 2 4661 

100 1000 10,000 
FREQUENCY IN CYCLES 

Figure 17. Spectra of two types of background 
used in masking repeated pulses. 

the remaining one had a power level spectrum 
given by curve B in that figure. The three back- 
grounds of relatively low frequency were re- 
cordings of heterodyned self-noise received in 
the supersonic gear of an S-class submarine; 
hence the shapes of all three spectra were very 
nearly identical, being determined essentially 
by the response characteristics of the receiving 
gear and of the recording and reproducing sys- 
tems. One of the three self-noise samples was 
obtained while the submarine was underway at 
a depth of 90 feet and a speed of 3 knots. The 
quality of this noise background was described 
as “smooth”; it apparently consisted almost 
entirely of ambient water noise. The second 
of the self -noise samples was obtained with the 
submarine moving on the surface at a speed of 


10 knots, with the hydrophone axis at a relative 
bearing of 270 degrees. The quality of this 
noise was described as “rough,” and consisted 
of scratches superposed upon a hissing sound 
of variable loudness. For short periods, only 
the hissing sound could be heard. The third 
sample of recorded self-noise was obtained 
while the submarine was lying-to on the sur- 
face, charging its batteries with diesels, and 
with most of its auxiliaries secured. This noise 
was also described as “smooth,” being fairly 
free of sharp impacts and consisting mostly 
of ambient water noise. This last sample of 
recorded self-noise was used in the signal chan- 
nel only and in those cases in which pulsed 
noise served as the signal to be recognized. 

The fourth background used was a film re- 
cording of the output from a thermal noise 
generator. The spectrum of this sound, as mea- 
sured at the input to the headphones, is repre- 
sented by curve B in Figure 17 ; its character 
was smooth. The shape of its spectrum is 
largely a reflection of the band widths of the 
noise channel and the mixing stage. Since the 
spectra of pulses were restricted in much the 
same way by the band width of the test ap- 
paratus, it was unnecessary to use filters after 
the mixing stage (for the sake of confining the 
pulse spectrum to the limits of the masking 
noise band) when this wide-band thermal noise 
sample was used as the masking background. 
However, this statement does not apply to 
studies in which the masking noise sample had 
the spectrum described by curve A; some of the 
anomalous results described below, in connec- 
tion with tests in which the masking back- 
ground was confined to the narrow band, seem 
to have been produced by the fact that the 
pulse spectrum extended beyond that of the 
masking noise. The wide-band thermal noise 
represented by curve B was also used in some 
of the tests in which noise pulses were studied. 
For such tests, two separate sections of the 
noise film recording were used in the back- 
ground and signal channels, so that there was 
no correlation between the time-amplitude pat- 
terns of the pulses and the masking background. 
The specific type of noise used in any case is 
indicated in the figures showing the results of 
the listening tests. 


EESTRTCTED 


202 


NOISE MASKING OF ECHOES 


Several properties of the presented signals 
and backgrounds may have influenced these 
tests. The deviations from absolute flatness 
shown by curve B may have affected, to some 
extent, the results obtained with pulses of dif- 
ferent intrinsic frequencies. Also, while the 
headphones used were of relatively high quality, 
they probably had a somewhat poorer response 
in the high-frequency region than in the low. 
Finally, it seems probable that, when the wide- 
band noise depicted by curve B was presented 
to the listeners at a comfortable loudness level, 
the components below about 200 cycles did not 
reach threshold (see Figure 79 in Chapter 
4) ; hence, the results of masking tests with 
pulses of very low intrinsic frequency may have 
been affected in part by the fact that the sig- 
nals were threshold rather than masking 
limited. 

Some of the tests were made with several 
observers participating. In the remainder, the 
results were obtained by a single observer in 
self-administered tests. This observer had par- 
ticipated in the group test as well, and his re- 
sponses were found to be reliable and very 
nearly typical of the group response. Two types 
of test were conducted. In one, the observers 
had no foreknowledge of the character and 
quality of the sound which was to be identified 
as the signal. As indicated below, results in 
this type of test were significantly different 
from the results obtained when the observers 
knew the nature of the signal in advance. 

In the various figures given in this section, 
experimental points are shown only for the one 
case in which they are available (Figure 18). 
The report describing the results of these tests^^ 
states that in no case were the experimental 
points more than 2 decibles away from the line 
drawn. 


8.5.2 Qf Pulse Repetition Frequency 

The effect of pulse repetition frequency on 
the audibility of 1-millisecond pulses is shown 
in Figure 18. In this case, pulses were repeated 
at regular rates shown on the horizontal scale 
in the figure. The ordinates represent recogni- 
tion differentials relative to the standard ref- 
erence band (0.1 to 10 kilocycles) corrected 
to a flat background spectrum. A glance at the 


observed spectrum of noise B indicates that, in 
fact, the spectrum of the masking background 
was not flat and that the equivalent rectangular 
noise band is much more nearly included be- 
tween the limits 0.2 and 6 kilocycles. Actual in- 
tegration of the total power contained in noise 
























< 

\ 



















\ 

s 



>251 111 1— —I — I— 

4681 2 4681 2 4681 2 4 

10 100 1000 
PULSE REPETITION FREQUENCY IN CYCLES 


Figure 18. Effect of pulse repetition frequency on 
recognition differentials. The signal consisted of 
1-millisecond 800-cycle pulses repeated at the rates 
indicated on the horizontal scale. The background 
was wide-band thermal noise. For comparison, the 
recognition differential for a single presentation of 
a 1-millisecond 800-cycle pulse masked by a 10- 
kilocycle noise band is 6.5 decibels (see Figure 8). 


B shows that the equivalent flat spectrum has 
a width of about 5,650 cycles. Hence the ob- 
served values of the signal-to-noise ratio at 
primaudibility have been diminished by 2.5 
decibels to convert them to recognition differ- 
entials for a flat noise background in the stand- 
ard reference band. This correction is some- 
what uncertain since the meaning of the data 
reported in reference 6 is not wholly clear. 

The circles in Figure 18 represent the ob- 
served points ; the heavy line has been drawn to 
fit these points as well as possible in a smooth 
manner. Figure 18 indicates that the audibility 
of multiple pulses does not change by more 
than 1 decibel as the pulse repetition frequency 
increases from 5 to 20 per second. With in- 
creasing rates of repetition the audibility im- 
proves; for a just audible string of pulses, the 
required signal intensity decreases by 10 to 13 
decibels for a tenfold increase in pulse repeti- 
tion frequency. This result may be quantita- 


UCDWR TESTS ON REPEATED PULSES 


203 


lively explained as a result simply of the in- 
crease in average signal level with increasing 
prf, if the response of the ear is assumed to 
depend primarily on the total energy in the sig- 
nal. Since the average signal energy is propor- 
tional to the number of pulses per second, this 
energy presented to the ear will increase 10 
decibels per tenfold increase of prf. The con- 
clusion that the masking of a sufficiently short 
pulse or group of pulses is determined prima- 
rily by the total signal energy presented during 
the ear’s integration time is in general agree- 
ment with the results already discussed and 
summarized in Section 8.1. 

One would expect that as the time interval 
between pulses approached zero and the pulses 
merged to form a sustained tone, the observed 
recognition differential would approach that 
observed for a sustained tone masked by wide- 
band noise. Since the pulse length used for the 
data in Figure 18 was 1 millisecond, the RD 
for a prf of 1,000 per second should equal the 
RD found with the sustained tone. Comparison 
between these two recognition differentials 
shows, in fact, very good agreement between 
them. The repeated pulse RD for a prf of 1,000 
per second, found by extrapolation in Figure 
18, is —23 decibels, referred to a flat noise band 
10 kilocycles wide. Since the critical band for 
an 800-cycle tone is 38 cycles wide, the com- 
puted recognition differential is 10 log 
38/10,000 or —24.2 decibels. The slight discrep- 
ancy between these two results may be due to 
the fact that, when an 800-cycle tone is sub- 
divided into 1-millisecond pulses, the period of 
the generated pulses is much more nearly that 
of a 1-kilocycle wave than it is of an 800-cycle 
wave. If this type of distortion actually played 
a part in present tests, a critical band width of 
50 cycles should be assumed in the preceding 
calculation, in which case the computed RD is 
in exact agreement with that derived from Fig- 
ure 18. 

There is, however, another way of looking 
at this problem. Mathematically, a string of re- 
peated pulses can be analyzed into a spectrum 
of sustained tones whose sum is equivalent to 
the string of pulses and is physically indistin- 
guishable from that string. It might be expected 
that such a string of pulses would be audible 
when one or more of the tones in the spectrum 


was equal in intensity to the noise level in the 
corresponding critical band. 

This type of analysis may be expected to be 
relevant to the ear only for values of prf 
greater than about 20 per second. With more 
widely spaced pulses, the ear will resolve the 
separate pulses and respond to them as units. 
For prf’s more than 20 per second, the sound 
has a high degree of roughness, but the succes- 
sive pulses are not heard as clearly defined 
individual units. For the higher values it is 
therefore relevant to investigate the intensities 
of the individual tones making up the spectrum 
of the repeated pulses and to compare these 
with the masking background. 

Unfortunately, the exact analysis of the 
strings used in these tests is somewhat com- 
plicated, since the square-wave modulation in- 
troduced by the gating circuit bears no simple 
relationship to the phase of the oscillator tone. 
With certain approximations, an analysis can 
be carried out, but the results are not easily 
reconciled with the observed masking data. In 
fact, the slope found on this approximate 
theory is roughly a 20-decibel decrease of rec- 
ognition differential for each tenfold increase 
of prf, since the intensities of the tones in the 
equivalent spectrum are proportional to the 
square of the prf. It is possible, however, that, 
because of their harmonic and phase relation- 
ships, the components of such a spectrum are 
integrated so that they become primaudible as 
a group. Since the number in such a group 
would be proportional to the prf, the observed 
slope of 10 decibels per decade would be ex- 
pected. More detailed study is required to in- 
dicate the relationships between the perform- 
ance of the ear and the spectrum of this type 
of signal. 

8.5.3 Effect of Pulse Type 

Observations were made of the masking of 
repeated pulses of various types with a pulse 
repetition frequency of 5.6 per second. Among 
the variables that were studied in this connec- 
tion were the pulse length and the frequency 
of the tonal pulse used. In addition, the recogni- 
tion of noise pulses was studied. 

Figure 19 shows the recognition differentials 
obtained with pulses of lengths between 1 and 


^-j^TRiCTED 


204 


NOISE MASKING OF ECHOES 


10 milliseconds and with frequencies of 200 to 
6,000 cycles. No significant difference in rec- 
ognition differentials, that is differences ex- 
ceeding the experimental error of 1 to 2 
decibels, appears to have been obtained by vary- 
ing the heterodyne frequency between 200 and 



Figure 19. Effect of pulse length on audibility of 
pulses repeated 5.6 times per second. Curve A is for 
200-cycle pulses masked by noise whose spectrum is 
given by curve B in Figure 17. Curve B is for pulses 
with frequencies between 0.5 and 6 kilocycles 
masked by noise (curve B in Figure 17). Curve C 
is for 700-cycle pulses masked by noise (curve A 
in Figure 17). 

6,000 cycles, although the 200-cycle data in the 
figure indicate a loss of detectability amounting 
to 3 to 5 decibels, and the 700-cycle data imply 
a gain of 6 to 7 decibels. 

It will be noted that the slope of curve B 
shown for the pulses of 0.5 to 6 kilocycles, 
masked by wide-band thermal noise, is 11 
decibels for each tenfold increase in pulse 
length. This is to be compared to the slope of 
10 decibels to be expected if the response of 
the ear to the short pulses is determined prima- 
rily by the total energy presented to the ear 
per second. This relatively close agreement 
provides added confirmation that the ear does, 
in fact, behave as an integrating mechanism 
for the masking of short pulses ; it has already 
been noted in Sections 8.1.2 and 8.4.4, however, 
that this is not in agreement with the behavior 


of the ear in estimating the loudness of a short 
pulse, and this apparent difference between the 
results of the loudness and the masking studies 
still remains to be explained. 

The data shown in curve A for a pulse fre- 
quency of 200 cycles and a masking back- 
ground of wide-band thermal noise are based 
on two pulse lengths, 5 and 9 milliseconds 
respectively. Shorter pulse lengths were not 
used since it was felt that these would contain 
too small a fraction of a cycle. The discrepancy 
between curves A and B is probably not real. 
For example, when a wide band of fiat thermal 
noise is presented at a comfortable listening 
level, the noise components at frequencies of 
200 cycles or lower are likely to fall below the 
absolute audibility threshold (see Figure 79 in 
Chapter 4) . More data would be required, how- 
ever, to give reliable results on this point. 

For comparison with the data on single 
pulses, the curves in Figure 19 have been tran- 
scribed to Figures 8 and 12. To express the ob- 
served recognition differentials in terms of a 
reference band 1 kilocycle in width, 10 decibels 
have been added to the ordinates shown in Fig- 
ure 19. The recognition differentials found in 
this way are then comparable to those discussed 
in Section 8.4. It will be noted that this curve 
lies about 6 decibels below that for the single 
pulses, but that its slope is comparable. Thus, 
it may be inferred that a repetition rate of 5.6 
per second gives an BD 6 decibels lower (more 
favorable) than a single pulse of the same 
length. This may be compared with a gain of 3 
decibels found in Section 8.1.3 from a single 
repetition of a pulse. 

For comparison with curve B, curve C in 
Figure 19 applies to 700-cycle pulses masked 
by rough self-noise with the spectrum given by 
curve A in Figure 17. The spectrum of the 
pulsed signal extended beyond the limits of the 
rather narrow noise band ; this is to be expected 
theoretically and was verified by narrow-band 
filter analyses of the signal spectra. Thus, 
detectability in the case of curve C was better 
than that in the case of curve B, because the 
pulse spectrum was wider than that of the 
noise. The results are not representative of any 
practical situation, in which the same receiving 
system would be used for both signal and noise. 
This observation illustrates the danger in- 




RESTRICTED 


UCDWR TESTS ON REPEATED PULSES 


205 


herent in tests in which the signal and noise 
are not passed through a common filter. It also 
indicates that, even for short pulses, the ear 
continues to act as a frequency analyzer and 
the masking effect of wide-hand noise comes 
primarily from those noise components with 
frequencies in the immediate neighborhood of 
the signal components. 

The results obtained in the study of thermal 
noise pulses are summarized in Figure 20. In 




Z 2 


-5 


-15 




















N. 


2 3 4 56789 10 

PULSE LENGTH IN MILLISECONDS 


Figure 20. Audibility of noise pulses (curve B in 
Figure 17), masked by a noise background (also 
curve B in Figure 17) . For comparison, the dashed 
curve, taken from Figure 19, shows recognition 
differentials for tonal pulses with frequencies be- 
tween 0.5 and 6 kilocycles. The pulse repetition fre- 
quency in both cases with 5.6 times per second. 


this figure the ordinates show recognition dif- 
ferentials, referred to the standard reference 
band, for pulses of the indicated duration, re- 
peated 5.6 times per second. The upper line 
gives the measured values for pulses of thermal 
noise, while the lower, transcribed from Figure 
19, represents the data for tonal pulses of fre- 
quencies between 500 and 6,000 cycles. It will 
be noted that the upper line shows a recogni- 
tion differential independent of pulse duration 
in the range from 1 to 10 milliseconds. The two 
lines, if extended to the left, would intersect at 
a pulse length of about 0.6 millisecond. 

Although the constant recognition differ- 
entials found for the thermal noise pulses are 


perhaps surprising, it is to be expected that 
this curve should show a slope less steep than 
that for the tonal pulses. It is evident, how- 
ever, that the two curves should approach 
equality for sufficiently short pulses. This re- 
sult follows either from the increasingly wide 
spectrum of a shorter and shorter pulse or from 
the observed fact that a very short pulse has 
no apparent tonality. Since a very short tonal 
pulse is indistinguishable from a very short 
noise pulse, the recognition differentials shown 
by the curves in Figure 20 should approach 
equality for short pulse lengths. On the other 
hand, the solid line should lie above the dashed 
when the pulse length is much increased. This 
follows from the fact that a distributed noise is 
less audible than a pure tone in the presence of 
a wide-band noise (compare the wide-band rec- 
ognition differentials in Figures 58 and 59 of 
Chapter 4). While a sustained tone at 1,000 
cycles can be heard 23 decibels below a fiat 
background 10 kilocycles wide, a noise similar 
in spectrum to the background and with no 
modulation could be at most 3 decibels below 
the background at primaudibility. Thus, the 
two lines in Figure 20 would be expected to 
diverge with increasing pulse length, the dif- 
ference between them approaching a maximum 
value of 20 decibels. The behavior shown in 
Figure 20 therefore appears reasonable. 

Data on masking of noise pulses by noise are 
also given in the next section for the situation 
where the narrow-band noise source shown in 
curve A of Figures 17 was used for both signal 
and background. These data give quite different 
results from those obtained with the wider 
band noise, and in particular show recognition 
differentials decreasing with increasing pulse 
length (see Figures 21 and 22) . However, these 
tests with narrow-band noise pulses are not too 
reliable, since pulsing the noise may have 
widened its spectrum, giving audible com- 
ponents at frequencies where the unpulsed 
noise background was weak. Hence, only the 
data on wide-band thermal noise will be con- 
sidered at this point. 

Although these data are neither sufficiently 
accurate nor measured over a sufficiently great 
range of pulse length to allow precise con- 
clusions, comparisons of these findings with 
other results are of interest. Since the spectra 


206 


NOISE MASKING OF ECHOES 


of signal and background in these thermal noise 
studies are nearly identical, the recognition dif- 
ferentials shown by the solid line in Figure 20 
give the ratio of signal to noise in each critical 
band at primaudibility. They may therefore be 
compared with the continuous curve in Figure 
11 which gives the signal-to-noise ratio in a 
critical band for a tonal pulse at primaudibility. 
This curve has been computed theoretically 
from the observed dashed curve, taking into 
account the increasing width of the pulse spec- 
trum with decreasing pulse length. It is per- 
haps significant that the computed critical-band 
RD increases by less than 2 decibels as the 
pulse length decreases from 10 milliseconds to 
1 millisecond. This agrees, to within the ex- 
perimental error, with the constant recognition 
differentials shown by the upper line of Figure 
20. Thus, it is possible that in this range of 
pulse lengths the critical band RD actually 
changes very little. Additional evidence for this 
tentative point of view is presented in Section 
9.2.2. 

Studies were also made of the effects pro- 
duced by passing the signal through filters of 
different widths. In all cases, however, the filter 
widths were at least as great as the reciprocal 
of the pulse length, and therefore passed most 
of the signal energy. While some distortion 
of signal envelope was produced, this effect 
should not be aurally noticeable at the short 
pulse lengths used. In agreement with expec- 
tation, the standard-band recognition differ- 
entials found with filters were found to be 
equal to those obtained without filters, to within 
the experimental error. Since these recognition 
differentials are expressed in terms of a wide 
masking band it follows that the primaudible 
signal level was unaffected by the filters used, 
although the level of the actual noise presented 
was of course affected by the filters. 

8.5.4 Qf Irregular Repetition Rate 

and Amplitude Modulation 

Figures 21, 22, and 23 show the effects of ir- 
regular as compared with regular rates of 
pulse repetition. The effect of this factor is 
shown for pulses of the indicated durations and 
frequency compositions and in the presence of a 
noise background with the spectrum shown by 


curve A in Figure 17. Both rough and smooth 
noise backgrounds were used. As indicated in 
the preceding section, this background noise did 
not adequately mask the pulses. Consequently, 
the ordinates in Figures 21 through 23 are 



Figure 21. Effect of random-interval pulsing for 
a 700-cycle tone, masked by rough self-noise (curve 
A in Figure 17). 



Figure 22. Effect of random-interval pulsing for 
smooth self-noise, masked by rough self-noise. 
Spectra of signal and background are given by 
curve A in Figure 17. 

arbitrary and do not represent recognition dif- 
ferentials. The results shown in the various 
graphs may, however, be used for comparing 
the effects of regular and irregular pulse 
repetition rates, and also smooth with rough 



UCDWR TESTS ON REPEATED PULSES 


207 


background noise types. Due to the spreading 
of the pulse spectra beyond the limits of the 
masking noise band, even these comparisons 
may not be truly representative of the results 
which would be obtained in the field. 

Figure 21 applies to a 700-cycle tone of the 



Figure 23. Effect of random- interval pulsing for 
smooth self -noise pulses, masked by a smooth self- 
noise background. Spectra of signal and back- 
ground are given by curve A in Figure 17. 

indicated duration pulsed at a regular rate of 
5.6 per second, and also pulsed in an irregular 
fashion. The apparent advantage of some 2 
decibels for irregular as compared with regular 
pulsing may be due to the fact that the rough 
self-noise was observed to have an approxi- 
mately cyclical time-amplitude pattern of its 
own. Possibly, therefore, the irregular pulsing 
was easier to detect than the regular because it 
could be more readily distinguished from the 
time pattern exhibited by the noise background. 
In other words, when hand keying is used, a 
signal rhythm should be selected which is as 
different from that of the noise as possible. It 
is also possible that differences in the average 
pulse repetition frequency may be in part re- 
sponsible for the difference between these two 
curves. 

Figure 22 depicts changes in performance 
due to regularity of pulsing for the case in 
which a signal obtained from smooth self-noise 
was detected in the presence of rough self- 
noise. Before pulsing, both of these noises had 
the characteristics shown by curve A in Figure 
17. The results given in this figure correspond 


more nearly to expectation than do those in 
Figure 21, since it might be expected that 
regular patterns could be more readily recog- 
nized with certainty than irregular patterns 
of the same pulses. In both cases, however, the 
RD for noise pulses decreases with increasing 
pulse length, in contrast to the data for wide- 
band noises shown in Figure 20. Similar re- 
sults were obtained when smooth self-noise was 
used as the masking background (see Figure 
23). This discrepancy between results found 
with wide-band and narrow-band noise has al- 
ready been discussed in the preceding section. 

The importance of time pattern is illustrated 
in Figures 24 and 25, which show the relative 
audibility of signal pulses obtained from 
smooth self-noise, and masked by either smooth 
or rough self-noise. In these figures, the curves 
for a rough noise background have been tran- 
scribed from Figure 22, and those for a smooth 



Figure 24. Effects produced by the time pattern 
of masking noise for smooth self -noise repeated 5.6 
times per second. The solid curve applies to a 
smooth self-noise background, while the dashed 
curve applies to a rough self-noise background. 
Spectra of signal and background are given by 
curve A in Figure 17. 

background from Figure 23. The greater mask- 
ing efficiency of the rough background noise 
shown in both cases was apparently associated 
with its time-amplitude pattern. Thus, when 
the rough masking noise was used, the observers 


208 


NOISE MASKING OF ECHOES 


had to listen for several seconds in order to be 
sure that a signal was present and to detect its 
rhythm (see also Section 5.5) . 


> UJ 

UJ m 

-j a 
-I > 
< q: 
z < 
<s> a: 

m t 

UJ e 

m < 
5 ^ 
5 o 

< (D 
5 < 



PULSE LENGTH IN MILLISECONDS 


Figure 25. Effects produced by the time pattern 
of masking noise for smooth self-noise pulses re- 
peated at random. The solid curve applies to a 
smooth self-noise background, while the dashed 
curve applies to a rough self-noise background. 
Spectra of signal and background are given by 
curve A in Figure 17. 


Another significant observation should be 
noted at this point concerning the effect on rec- 


ognition differentials of advance familiarity 
with the presented signal. It was generally ob- 
served that a just audible signal of unknown 
character was 3 to 8 decibels higher than a sig- 
nal of known character when irregular pulsing 
was used; in the case of regular pulsing, the 
RB for unknown signals was 3 to 5 decibels 
above that for known signals. The report de- 
scribing the results® indicates that the magni- 
tude of this effect should be considered the 
same for regular and irregular pulsing, in view 
of the experimental uncertainties. For com- 
parison, it will be recalled that similar tests 
with ship signals (see Section 4.1.5) indicate 
that the primaudible signal-noise ratio for un- 
known signals is 2 to 3 decibels higher than for 
familiar signals. It was the opinion of the ob- 
servers in these tests that strings of very short 
pulses would not attract attention at an enemy 
listening location unless they were extremely 
loud with respect to the background or unless 
they suddenly increased in loudness as the bear- 
ing of the enemy receiver was changed. 


^/restricted 


Chapter 9 


REVERBERATION MASKING OF SIGNALS WITHOUT DOPPLER 


R ealistic tests on masking by reverberation 
are considerably more difficult to perform 
than on masking by noise, since the character- 
istics of reverberation are so varied and so 
complicated. As a result of this complexity of 
reverberation, it is not feasible to use simulated 
reverberation in masking tests, since no method 
of simulation has been devised which produces 
sound nearly enough identical with observed 
reverberation. Instead, all studies of this 
subject have used reverberation background 
received in sonar gear, either presented directly 
in a test or recorded for subsequent presenta- 
tion. The signals used have included injected 
pulses as well as recorded echoes. In almost all 
cases, one of the primary difficulties has been 
the narrow frequency band to which CW rever- 
beration is confined. As a result of this small 
frequency spread in the masking background, 
it is not easy to avoid spurious effects resulting 
from the finite width of the signal spectrum. 
There are a number of instrumental effects 
which can broaden the spectrum of a pulse or 
even of a recorded echo beyond the spectrum 
of the reverberation, and much of the effort in 
these tests has been devoted to the elimination 
of this effect. 

9 1 BELL TELEPHONE LABORATORIES 
TESTS 

In a pioneering study,i masking experiments 
were made by BTL with pulses and recordings 
of real echoes both of which were mixed with 
recorded CW reverberation samples. The pulse 
signals were rounded as well as rectangular. 
Both auditory and visual detection were stud- 
ied, but only the auditory results are described 
here. 

General Procedure 

The reverberation background used in these 
tests was a playback of disk recordings. Each 
disk contained about 100 samples of volume re- 


verberation, produced in deep water by a CW 
transmission 5, 25, or 100 milliseconds long 
and received at bearings of 90 or 270 degrees 
by a vessel moving at about 8 knots. The fre- 
quency put into the water was 24 kilocycles, 
and the received reverberation was heterodyned 
to 800 cycles and transmitted by an FM link 
to a shore station where it was recorded. Suc- 
cessive reverberations in each recording were 
spaced 4 to 6 seconds apart. 

Owing to overloading of the receiving-record- 
ing system, the initial portion of each rever- 
beration was of nearly uniform amplitude and 
had appreciable harmonic content. This part 
was followed by a second portion in which the 
sound level fell more or less rapidly and was 
featured by rapid fluctuations of amplitude, or 
blobs, with durations approximately equal to 
that of the emitted ping. The third section of 
each recorded reverberation was of relatively 
low level, approaching that of needle scratch 
and other noises incidental to the recording and 
playback. Signals were mixed with these back- 
grounds in such a way that the signal occurred 
during the second, strong, but decaying, portion 
of the reverberation samples. To minimize an- 
noyance and confusion which might be pro- 
duced by the playback noise associated with the 
third segment of the reverberation samples, a 
3,500-cycle low-pass Alter was included in the 
background channel. 

The reverberations were recorded at bear- 
ings of 90 or 270 degrees in order to eliminate 
the effects of own-doppler as much as possible. 
However, careful examination of recorded re- 
verberation samples indicates fairly large 
variations in pitch of the reverberation are not 
uncommon. Such pitch changes may be ex- 
pected to influence recognition differentials ; 
hence it is desirable to eliminate from masking 
tests all reverberation samples which show this 
tendency. While no selection of the reverbera- 
tion samples for the purpose of eliminating 
variable pitch items was made in the tests 
under discussion, comparison with other ob- 


210 


REVERBERATION MASKING OF SIGNALS WITHOUT DOPPLER 


servations (see Figure 12) implies that insta- 
bility in pitch of the reverberation background 
probably played no very large role in deter- 
mining the overall results obtained in these 
tests. Further study will be required before it 
is possible to decide whether systematic changes 
of reverberation frequency with time are im- 
portant in practice. 

Either pulse or echo signals were injected 
at the same range in each case; that is, the 
length of time by which the signal followed the 
start of each reverberation was fixed. This 
procedure involves the risk of introducing un- 
realistic practice effects by conditioning the 
observers to concentrate on the signal at a par- 
ticular instant during the course of each re- 
verberation and to disregard misleading blobs 
which may occur earlier or later in the sample. 
However, evidence discussed in Section 9.2.2 
indicates that the probable magnitude of such 
an effect in systematically diminishing rec- 
ognition differentials probably did not exceed 
1 to 2 decibels. 

The sounds used were presented individually 
to the members of the test group, usually 4 to 6 
in number. Each test contained 100 reverbera- 
tion samples (100 listening intervals). No 
more than two such test sequences were pre- 
sented to any observer at a single sitting; and 
the factors tested in two such sequences were 
selected, from among those included in the 
overall program, in as random and unrelated 
a manner as possible. 

Before the tests were begun, a number of 
subjects were given instruction and practice in 
masking tests of the kind under study. All the 
observers actually used in the study under dis- 
cussion were drawn from the more able group 
of the subjects examined, on the assumption 
that more uniform data would be obtained and 
that there would be less practice effect to 
modify the performance of the abler group 
with time. Later tests indicated that the magni- 
tude of the practice effect was probably less 
than the variation in performance of a single 
individual in successive tests. Checks with 
groups of 5 to 6 observers indicated that the 
results obtained in a single test given to the 
group were reproducible to within about 2 
decibels. 


During each sequence of 100 samples, the 
level as well as the range of the injected signal 
was held fixed. The observers could adjust the 
level of the presented sounds for maximum 
comfort; thus, the gain settings chosen varied 
somewhat among different observers. Approxi- 
mately 50 signals were injected at random in 
each sequence of 100 listening intervals. In suc- 
cessive sequences, the gain in the background 
channel was left unchanged at a constant value, 
and the level of the injected pulse was varied 
in steps of 2 to 3 decibels from a value produc- 
ing about 95 per cent recognition probability 
to a value giving approximately 50 per cent 
recognition probability. The highest levels were 
used first, in order to minimize practice effects 
and to establish listener confidence. In addition, 
competition was fostered among the observers 
in order to relieve the monotony of the task. 
Just before each test, the observers were per- 
mitted to listen to as many signal-reverberation 
mixtures as they desired and at the same signal 
level to be used in the forthcoming test. In the 
report describing these tests, ^ it is stated that 
“for some of the more difficult tests [that is, 
lower signal levels], and at the request of the 
observers, the practice was begun with a higher 
signal level which was gradually reduced to 
the test value. This procedure often improved 
considerably^ the observers’ ability to detect the 
more obscure signals. If, after practice, the ob- 
server felt that for any reason his performance 
was not up to par, testing with that individual 
was postponed. The observer was also made to 
feel free to stop testing at any time he desired. 
While these privileges were seldom exercised, 
they helped to relieve any tendency toward 
nervous strain and thus contributed to better 
and more nearly uniform results.” 

General observations made in reverberation 
masking programs indicate that the cues are 
often elusive and that experience and favorable 
test conditions do have an important influence 
on the character of the results obtained. It 
should be noted, however, that such test proce- 
dures tend to define the upper limit of per- 
formance and may not be directly applicable to 
the more complicated and less favorable set of 
conditions typically met in practice. 

All tests were conducted in a soundproof 
room, and loudspeaker presentation was used 


lESTKlClEI) 


BELL TELEPHONE LABORATORIES TESTS 


211 


exclusively. One of the loudspeakers used was 
a regulation Navy model; the other was of 
higher quality, having good response over a 
broader band than the first and passing more 
of the low-frequency sounds, that is, it was less 
sharply tuned than the first. 

The listeners expressed their judgments of 
signal audibility by pressing a telegraph key, 
which actuated recording circuits designed to 
count separately all errors of omission and 
commission. Failure to depress the key within 
a 2-second interval beginning with the signal 
was scored as an omission, while a vote at any 
other time was registered as a commissive 
error. 

The signal-to-reverberation ratio in any test 
was defined as the ratio of the average instan- 
taneous power of a signal to that of the rever- 
beration during a period equal in length to 
that of the signal and coincident with it. When 
the signal was a pulse, its power was readily 
measured; the method of measuring the power 
of recorded echoes is described in Section 8.4.6. 
Since the reverberation level (at the instant 
the signal was injected) varied from one back- 
ground sample to the next, a statistical average 
of the 100 reverberations in each test was used 
for the purpose of defining reverberation 
power. To do this, the power-time trace for 
every fifth reverberation on a disk (20 in all, 
for each disk) was obtained with a power level 
recorder whose rated writing speed was 360 
decibels per second. The power measurements 
for the 20 samples selected were averaged at 
l^-second intervals, and the arithmetical means 
of these individual readings were used in com- 
puting signal-to-reverberation ratios.^ The 
standard deviations from such averages varied 
somewhat from disk to disk, ranging from 1 
to 3 decibels. This implies that the extremes of 
fluctuation extended over a total range of about 
18 decibels. 

The finite response times of power level 
recorders tends to smooth out the extremes of 
fluctuation because the instrument shows the 
integrated power fed to its terminals during 
its integration time. The effect upon calculated 


^ For purposes of comparison, the average power 
level (the geometrical mean) was also computed and 
was found not to differ significantly from the average 
power. 


recognition differentials of various methods of 
measuring the level of reverberation back- 
ground is discussed in some detail in Section 
9.2.2, where it is indicated that the method just 
described is perhaps as satisfactory as any when 
the purpose of the measurements is to obtain 
predictions of probably performance in the field. 
The report describing the tests discussed in the 
present section^ assigns an estimated value of 
less than 1 decibel to the net effect of all sources 
of objective error, such as faulty calibration or 
instability in the test apparatus. 

Recognition Differentials for 
Injected Pulses 

In the first set of tests, pulses were generated 
by an 800-cycle oscillator in the signal channel. 
The harmonic content of this oscillator tone 
was more than 40 decibels below the funda- 
mental. A gate circuit, or electronic relay, was 
used to generate rectangular pulses 5, 25, and 
100 milliseconds long. These pulses displayed 
no noticeable switching transient, and, in the 
masking tests, were mixed with reverberations 
produced by transmissions of the same dura- 
tion as that of the pulse signal. The evidence 
discussed in this section indicates that the 
envelopes of these artificial pulses probably 
had somewhat squarer corners than those used 
to produce the reverberations, that is, the sig- 
nal spectra contained stronger side-band fre- 
quencies than did the background spectra. 

Rectangular Pulses 

The results obtained in masking tests with 
extremely square-cornered pulses are listed in 
Table 1. The tabulated percentages refer to the 
fractional number of injected signals which 
were correctly identified. Commissive errors 
were nearly negligible, averaging about 3 per 
cent in all tests to which reference is made in 
this table, and such errors showed no signifi- 
cant dependence on pulse length or signal-to-re- 
verberation ratio.' It is generally agreed that 
the results quoted in Table 1 are anomalous be- 
cause the test conditions deviated from those 
which exist in the field. These data are never- 
theless worth examining because they help to 


0 (kkstki(:if.i) ^ 


212 


REVERBERATION MASKING OF SIGNALS WITHOUT DOPPLER 


define the factors involved in the auditory 
detection of echoes masked by reverberation. 

The results obtained in these tests are 
atypical in a number of ways. Thus, as shown 


its spectrum extended beyond the masked 
region. In practice, the frequency compositions 
of reverberation and of an echo without doppler 
are probably closely similar ; hence, it does not 


Table 1. Aural recognition of rectangular 800-cycle pulses. 



Recognition probability in per cent 

Pulse-to- 

100-millisecond pulse 

25-millisecond pulse 

5-millisecond pulse 

reverberation 







ratio in db 

High-quality 

Service 

High-quality 

Service 

High-quality 

Service 


speaker 

model 

speaker 

model 

speaker 

model 

- 2 

97 

96 

90.5 

89 

97 

93 

- 7 

98 

84 

83 

82 

96.5 

87 

-12 

71.5 

60 

73.5 

75 

79 

67.5 


by Table 1, performance was as good for the 5- 
millisecond pulse as for the 100-millisecond ; in 
fact, ability to detect the signal is seen to be 
improved slightly by diminishing pulse dura- 
tion. In addition. Table 1 implies that tonal 
pulses can easily be recognized when they are 
more than 12 decibels below the level of the 
reverberation centered at the same frequency. 
Such a finding would not be surprising for the 
masking of one sustained tone by another of 
nearly equal frequency, because of the assis- 
tance rendered to the ear by the occurrence of 
beats. It is obvious, however, that if as many 
as 3 beats occur within the active duration of a 
5-millisecond pulse, the rate of beating would 
be about 600 per second. Such rapid beats are 
not very audible (see Figure 16 in Chapter 2). 
Furthermore, this would imply that the intrin- 
sic frequencies of the pulse and the reverbera- 
tion differed by about 600 cycles. Observations 
made during these tests, however, show that 
the intrinsic frequencies of the pulses and the 
recorded reverberation samples were nearly 
identical. 

It is therefore believed that the source of the 
high degree of signal detectability in these tests 
was the fact that the signal pulse was much 
squarer than that used to produce the rever- 
beration. Thus, the pulse spectrum contained 
stronger side bands than did the reverberation 
spectrum. Consequently, the pulse could be 
heard at such surprisingly low levels because 


seem possible to exploit this observation in 
order to improve performance when reverbera- 
tion is limiting in the field. 

The interpretation of the rectangular pulse 
data just outlined is consistent with the ob- 
servation that the subjects could hear a sharp 
hiss or click whenever the square pulse was 
injected, even at very low signal-to-reverbera- 
tion ratios. The effect of the disparity between 
the widths of pulse and reverberation spectra 
was intentionally emphasized in order to test 
this explanation. This was done by placing a 
140-cycle band-pass filter in the reverberation 
channel, in order to restrict further the width 
of its spectrum. Under these circumstances, the 
observers were able to detect a 100-millisecond 
pulse when its level was 26 decibels below the 
level of the 100-millisecond reverberation back- 
ground. These results are consistent with the 
previously expressed view that the ear continues 
to act as an analyzer even for sounds of very 
short duration and may also explain why the 
performance with the high-quality loudspeaker 
was somewhat better than with the standard 
Navy speaker. Since all of the other tests in the 
series under discussion were conducted with a 
high-quality loudspeaker exclusively, they do 
not indicate whether the response of speaker or 
headphone makes a significant difference in the 
practical case (when signal and reverberation 
have spectra of nearly equal widths) . The very 
gradual slope of the transition curves partly 


BELL TELEPHONE LABORATORIES TESTS 


213 


defined by Table 1 is probably related to the 
multiplicity of cues (loudness, frequency, en- 
velope, and others) , so that the observers did not 
use the same standard of judgment at all signal 
levels. 

Rounded Pulses 

In order to eliminate the spurious cues sup- 
plied by the strong side bands in the spectra of 
the rectangular pulses, the signal-reverbera- 
tion mixture was transmitted through a band- 
pass filter inserted between the mixer and the 
high-quality loudspeaker. The results obtained 
in this way are in fairly good agreement with 
those of other tests (see Figure 12) and proba- 
bly furnish a reliable guide to the performance 
which may be expected in practice. 

The band-pass filter used was centered at 
765 cycles. Frequencies 35 cycles above or be- 
low this midfrequency were attenuated to the 
extent of 2 decibels by the filter, and frequen- 
cies removed 70 cycles from the midpoint were 
attenuated by 10 decibels. Since pulse and 
reverberation spectra were centered at 800 
cycles, they did not coincide exactly with the 
midpoint of the filter pass band. Pulse and re- 
verberation levels were measured in the sig- 
nal and background channels. It would proba- 
bly have been preferable to make these mea- 
surements at the filter output, but the available 
evidence suggests that the recognition differ- 
entials obtained would not have been sub- 
stantially modified by this change. The effect 
of the filter in rounding the envelopes of both 
signal and reverberation is illustrated in Fig- 
ure 1. 

Pulses both 100 and 25 milliseconds long 
were studied with the aid of the band-pass 
filter. The 5-millisecond tests could not be re- 
peated, since the essential spectrum of such a 
short pulse is 400 cycles wide and thus exceeds 
the pass band of the filter. It is possible, also, 
that the 25-millisecond data obtained with the 
filter are somewhat less reliable than are the 
corresponding 100-millisecond data. In the 
former case, the essential spectra of signal and 
background are 80 cycles wide; since the mid- 
frequency of these sounds was approximately 
35 cycles above the midpoint of the filter, the 
envelopes of the 25-millisecond pulses may have 
been excessively distorted. 


The results obtained in the masking tests 
with the filter are shown in Figure 2. The 
upper thin lines represent the relative number 
of pulses correctly identified. Since use of the 
filter increased the resemblance between signal 
and background and since the observers were 



WITHOUT FILTER 


WITH FILTER 

Figure 1. Effect of the filter on pulse and rever- 
beration envelopes. Time scales of the pulse en- 
velopes, at the right in both cases, are expanded 
relative to those of backgrounds, which are shown 
at the left. 

encouraged to adopt a responsive rather than 
a cautious attitude, the number of commissive 
errors increased fairly rapidly with diminish- 
ing signal-to-reverberation ratio, approaching 
15 to 20 per cent for the lowest ratios shown 
in Figure 2. In the practical case observers are 
usually trained to be cautious, with the result 
that commissive errors are generally much less. 
The observed recognition probabilities have 
been corrected for this effect. The correction 
used here is, of course, only an approximation 
to the actual situation but probably represents 
the essential features of the problem. 




^ESTmCTED^ 


214 


REVERBERATION MASKING OF SIGNALS WITHOUT DOPPLER 


To derive this correction, the recognition 
probabilities for a cautious and uncautious ob- 
server may be considered, when each is pre- 
sented with signals of constant level in the 
presence of successive reverberations. Let Pu 
be the recognition probability for the un- 



Figure 2. Detection probability for 25- and 100- 
millisecond pulses masked by reverberation. Upper 
curves give raw scores; lower curves show the 
points, corrected for guessing. 

cautious observer, defined as the number of 
times that a presented signal is reported heard, 
divided by the number of times the signal is 
presented, and multiplied by 100 to give a per- 
centage. Let Pc be the corresponding quantity 
for the cautious observer. When a blank inter- 
val is presented, the cautious observer will in 
general report no signal. The uncautious ob- 
server will, however, report a signal for q per 
cent of the blank presentations. 

To obtain a relation between Pc, Pu, and q, 
the observed fact may be used that a signal 
which is reported by a cautious observer will 
usually also be reported by the uncautious ob- 
server. Thus, the difference between the two 
observers will be that the uncautious one will 
claim to have heard some fraction of the pre- 
sented signals which the cautious observer did 
not report. The simplest assumption is that the 
uncautious observer will report the same frac- 
tion of these relatively weak signals as he will 
of nonexistent signals, that is, that he will also 
claim to hear q per cent of the presented signals 
not reported by the cautious observer. Since 
the fraction of presented signals not reported 
by the cautious observer is 1— ( 2 ?c/ 100 ), we 
have the simple equation 

■p^ = ■pc+ (/ (1 - 


which may be transformed to yield 
1 - ( 9 / 100 ) ■ 

This equation has been used here to give cor- 
rected values of the recognition probability 
corresponding to what a hypothetical cautious 
observer would obtain. It may be noted that 
when results are based exclusively on guessing, 
the observed percentage recognition probability 
Pu becomes equal to q, and the corrected rec- 
ognition probability Pc vanishes. By use of this 
formula, the lower set of curves has been 
drawn in Figure 2 to give the corrected values 
of the recognition probability. In Figures 3 and 
6 only corrected curves are given, with the ob- 
served values of omitted. It will be noted 
that, as Pu approaches 100 per cent, Pc ap- 
proaches Pu ; this is partly the result of decreas- 
ing q with increasing signal-to-reverberation 
ratio and partly the diminished importance of q 
in correcting Pu as Pu increases. 

The values of n for the corrected transition 
curves shown by the lower lines in Figure 2 is 
about 1.7. This is somewhat smaller than is 
typical for tests in which guessing is penalized 



Figure 3. The detectability of 100-millisecond 
pulses masked by 25-millisecond reverberation is 
shown by the middle curve. The other curves have 
been transcribed from Figure 2 for comparison. 

Of these, the left hand curve refers to recognition 
of a 100-millisecond pulse masked by 100-milli- 
second reverberation, and the right hand curve to 
recognition of a 25-millisecond pulse masked by 
25-millisecond reverberation. 

(see Figure 10). The more gradual decrease of 
detection probability in the present tests may 
be due in part to the fact that a confident ob- 
server is more likely to identify correctly weak 
signals to which a cautious listener would not 
respond. 


BELL TELEPHONE LABORATORIES TESTS 


215 


The middle curve in Figure 3 shows the re- 
sults obtained by presenting a 100-millisecond 
pulse mixed with 25-millisecond reverberation. 
Both sounds were passed through the filter 
already described. This form of test indicates 
one method by which it may be possible to 
segregate the effects due to different variables 
and thereby to arrive at a better understanding 
of the factors which determine performance. In 
addition, this test resembles the type of situa- 
tion which may occur in practice when an echo 
is prolonged as a result of reflection or multi- 
ple-path transmission. It would also be interest- 
ing to determine the audibility of a pulse 
shorter than that used in producing the rever- 
beration. 

The audibility of a 100-millisecond pulse in 
the presence of 25-millisecond reverberation 
was about 6 decibels better at all signal-to- 
reverberation ratios than for a 25-millisecond 
pulse masked by 25-millisecond reverberation. 
Some such improvement would be anticipated 
from the increase of pulse length. In fact, it 
might be expected that, for a 100-millisecond 
pulse, performance would be better against a 
background of 25-millisecond reverberation 
than against 100-millisecond reverberation, 
since the difference in spectra, as well as the 
relative duration, of the rounded 100-millisec- 
ond pulse and the 25-millisecond reverberation 
blobs should facilitate recognition of the pulse. 
Actually, the observed results are some 3 
decibels worse at all signal-to-reverberation 
ratios than for a 100-millisecond pulse masked 
by 100-millisecond reverberation. Without 
further experimental work, it is impossible to 
say whether this effect is real and associated 
with differences in reverberation structure, or 
the result of some systematic error, perhaps 
associated with the effect of the band-pass filter. 

^ Recognition Differentials for 

Recorded Echoes 

To test the practical reliability of the results 
obtained with rounded pulses, a study was 
made of the audibility of recorded echoes. 
These were obtained by echo ranging on an 
artificial target with a pulse of 24-kilocycle 
sound 100 milliseconds long. The echoes and ac- 
companying reverberations were heterodyned 


to 800 cycles and recorded. The target used for 
this purpose was a triplane, consisting of three 
mutually perpendicular planes. This object has 
a very high target strength for its size, because 
a beam of parallel rays tends to remain parallel 
after reflection from it. Unlike a submerged 
submarine, the triplane can readily be kept at 
a fixed distance from the echo ranging vessel 
and is therefore much more convenient to use 
in testing work. 

The echo from a triplane is usually a fairly 
accurate reproduction of the outgoing pulse, in 
contrast to echoes returned by large targets, 
since such echoes tend to be considerably pro- 
longed and frequently show marked amplitude 
modulation not present in the outgoing pulse. 
Thus, these tests do not represent what would 
be obtained with actual echoes from submarine 
targets. The purpose of the tests was pri- 
marily to provide recognition differentials for 
pulses and reverberation produced with exactly 
the same gear at a single time, and thus to 
eliminate the possible errors introduced by use 
of reverberation produced with one source and 
pulses produced with another. 

In obtaining the echoes, the triplane was 
placed at moderate ranges, where the rever- 
beration was weak. The echoes stood out 
strongly above the accompanying background. 
Subsequently, these echoes were recorded on 
another disk with the gain varied in such a 
way that a series of echoes of nearly equal 
amplitudes was obtained. 

In cutting the records which were to contain 
only the echoes, a gate circuit was used to ad- 
mit the echo. This gate was tripped by the be- 
ginning of each reverberation. Since the time 
interval between the beginning of the rever- 
beration and the occurrence of the echo varied 
somewhat over the course of the original rec- 
ord, it was necessary to adjust the “open’’ time 
of the gate to a value considerably longer than 
that of the echo in order to transmit each echo 
completely. Hence, all recorded echoes were 
preceded and followed by short intervals of 
reverberation. Figure 4 shows the envelopes of 
these reverberation remnants when the re- 
recording was made using a “square” gate, and 
Figure 5 shows the results obtained with a 
“round” gate. It will be noted from these fig- 
ures that the envelopes of the echoes are 


O ^ESfRlCTOD 


216 


REVERBERATION MASKING OF SIGNALS WITHOUT DOPPLER 


changed somewhat by either method of re- 
recording. 

The reverberation disk was also rerecorded 
from the original, in such a manner that it 
contained only reverberation and was devoid 


Figure 4. At the left is a CRO trace of an echo- 
reverberation mixture. To the right is shown the 
square-gate “echo” used to produce the mixture. 

of echo. This was done by blanking out that 
portion of the original trace which contained 
the echo and also all subsequent reverberation. 
This change was of no consequence, since in the 
tests the echo was injected at a considerably 


Figure 5. At the left is a CRO trace of an echo- 
reverberation mixture. To the right is shown the 
round-gate “echo” used to produce the mixture. 

shorter range than that at which it occurred 
in the original record. Because the weak ter- 
minal portion of the reverberation samples was 
blanked out, it was unnecessary to use the 3,500- 


cycle low-pass filter to eliminate the effects of 
record noise over the weak portions of the 
reverberation. 

To reproduce the echo and reverberation 
during the masking tests, the echo disk was 
played in synchronism with the mated rever- 
beration disk from which the echoes had been 
taken. The time interval between successive 
echoes in such a series was adjusted in such a 
manner that each echo would then coincide 
with the particular reverberation sample from 
which it had been obtained, although at a 
shorter range. This procedure was adopted in 
order to minimize spurious deviations from the 
practical situation, such as are associated with 
slow drifts of heterodyne frequency and other 
types of unstability which might supply false 
cues in the listening tests. Since the mated 
disks were cut from one original and were 
played at the same speed, the range relation be- 
tween echoes and reverberations could be fixed 
at the outset by means of an angular scale and 
did not vary appreciably during a given play- 
back. 

The first result of this program of measure- 
ments was the marked dependence of recogni- 
tion differential on the type of gating circuit 
used. The echoes which had been taken off the 
original record by use of the square gate were 
audible at a level 7 decibels lower than the cor- 
responding pulses rerecorded by use of the 
round gate. This finding is apparently the re- 
sult of the steep wave front transients pro- 
duced when the square gate was used. These 
transients, which resulted from the weak re- 
verberation before and after the echo, gave 
audible components at frequencies where little 
masking background was present. As expected, 
it was found that passing both the signal and 
the reverberation through a 140-cycle filter 
eliminated this spurious cue; thus, when this 
filter was used, the audibility of the trans- 
mitted echoes was very nearly equal to that of 
the rounded pulses (see Figure 2). 

The detailed results obtained with the round- 
gate echoes and with the square-gate echoes 
plus filter are summarized in Figure 6. In this 
figure the echo recognition probability in per 
cent is plotted against the echo-to-reverbera- 
tion ratio in decibels. The continuous line in 
this curve represents results obtained for in- 










BELL TELEPHONE LABORATORIES TESTS 


217 


jected pulses of 100 milliseconds duration, dis- 
cussed in the previous section (see Figure 2). 
It is evident from this figure that the points ob- 
tained when the round-gate echoes, with or 



Figure 6. Effect of the gate and filter on the 
detectability of recorded echoes. The curve is tran- 
scribed from Figure 2 and is seen to fit the points 
moderately well. 

without filter, were used agree well with the 
corresponding results found with the square- 
gate echoes, plus filter, and that both of these 
agree with the data for artificial pulses. 

UCDWR TESTS 

To obtain detailed information on the mask- 
ing of echoes by reverberation under a wide 
variety of situations, an extensive program 
was undertaken during the war by UCDWR. 
The objective of this program was primarily 
to determine recognition differentials of prac- 
tical importance for echoes masked by rever- 
beration under controlled and varied condi- 
tions, and secondly to cast light on the basic 
factors affecting echo masking. The chief vari- 
able of interest was the pulse length, since this 
quantity can readily be varied in the field to 
give optimum results. Other variables investi- 
gated were the range at which the echo ap- 
peared, the loudness level, and the amount of 
distortion introduced. The general procedure 
followed in this program is described in Section 
9.2.1, while the results obtained are given in 
Section 9.2. 2.*" 


' General Procedure 

These masking tests were made with re- 
corded beam-aspect echoes (no doppler) and 
recorded CW reverberations produced by pings 
of the same duration as that of the echoes to be 
detected. The 50 per cent recognition differen- 
tial for each test was found by plotting a tran- 
sition curve giving percentage recognition ob- 
tained by a group of five observers for each 
echo-to-reverberation ratio. 

Echo and Reverberation Recordings 

The echoes were obtained from an S-class 
submarine at close range. The sound-in-the- 
water, for reverberations as well as echoes, had 
a frequency of 24 kilocycles and was hetero- 
dyned to 800 cycles before being recorded on 
film. Recorded echoes were selected for this 
study which were fairly clean and which stood 
out well above the accompanying reverbera- 
tion. Sample echo recordings are shown in 
Figure 9 of Chapter 8. Furthermore, all the 
echoes used had essentially the same duration 
as the emitted pulse, as established by measure- 
ments made on the film. Transmission lengths 
of 11, 36, 97, 114, and 271 milliseconds were 
employed to obtain the echoes and reverbera- 
tions studied in the present tests. Some of these 
same echoes were also used in the thermal noise 
tests described in Section 8.4.6. 

The echo, together with accompanying back- 
ground, was recorded on the sound track of 
motion picture film. The remaining portion of 
the film, normally reserved for photographs, 
remained blank and was used to control the 
gating circuit which admitted the echo to the 
signal channel. By applying opaque tape to the 
blank portion of the film, except between the 
points corresponding to the beginning and end 
of an echo, and designing a photoelectric cir- 
cuit to actuate the signal channel only during 
the course of the echo proper, it was found 
possible to eliminate spurious cues due to tran- 
sients. Clearly, such recorded echoes are not 
entirely free from noise and reverberation 
background, but it is probable that contribu- 
tions from these sources had a negligible effect 
on the results. 

In order to minimize the effects of noise in- 


^ All the descriptive material in this section has been 
informally communicated. A report on these tests is 
expected to appear in the near future. 


f ^rSTRtctEn ^ 


218 


REVERBERATION MASKING OF SIGNALS WITHOUT DOPPLER 


herent in the recording processes, recordings 
were made at a fairly high gain setting; it is 
possible that some rounding of the echo and 
reverberation envelopes occurred whenever the 
received sound level exceeded the dynamic 
range of the recording equipment. Annoyance 
and fatigue due to film noise were virtually 
eliminated protecting the recording from 
dust and scratches. As a further precaution, 
the echoes and reverberations were passed 
through a common band-pass filter, with cut- 
offs at 565 and 1,130 cycles, respectively. This 
filter was wide enough to transmit the sounds 
without substantial distortion of their enve- 
lopes. Tests with and without the filter gave 
the same recognition differentials for given 
signal-background combinations. 

The sensed quality of the longer echoes was 
markedly tonal, and they had a definite and 
steady pitch. The shorter echoes were less tonal 
and more “noiselike”; thus, echoes with dura- 
tions of 10 milliseconds or less tended to sound 
like a crack or pop. 

Recordings were made of fairly long deep- 
sea reverberations. The dynamic range of the 
receiving system and the film, and the low level 
of inherent noise which can be achieved, per- 
mitted high-fidelity recording of reverberation 
background between ranges of about 100 yards 
and ranges exceeding 2,500 yards. Since the 
reverberations were recorded without any time 
variation of gain, the lower limit of 100 yards 
was imposed by overloading of the film for this 
and shorter ranges. This is a realistic facsimile 
of conditions encountered when using practical 
receivers which are not equipped with level 
stabilizers, such as rev erheration-contr oiled 
gain [RCG] or time-varied gain [TVG]. The 
upper limit of about 2,500 yards was imposed 
by the level of prevailing deep-sea ambient. 
In the tests described in this section, the gain 
in the background channel was held fixed dur- 
ing the course of each reverberation so that 
the average level of background was initially 
fairly high and gradually diminished. Figure 
7 shows oscillograms for two reverberations 
produced by CW transmissions of the indicated 
duration. Owing to the expanded time scale 
used in preparing the oscillograms, only short 
sections of these reverberations are repro- 
duced. In addition, it should be noted that the 


oscilloscope shows reverberation amplitude on 
a linear deflection scale, whereas the power 
level traces (Figure 5 in Chapter 7) show 
reverberation intensity on a decibel scale. 



Figure 7. Much-reduced CRO traces of CW rever- 
beration over a 300-yard range interval (0.38 
second) . The horizontal timing marks, visible below 
the reverberation traces, were used to determine 
the ranges at which echoes were injected and the 
corresponding reverberation levels. 

Between successive reverberation recordings, 
the output frequency of the receiver was varied 
in 20-cycle steps, and a great many samples 
were recorded, in order to provide an adequate 
supply of backgrounds which would match the 
pitch of the selected echoes and which would 
show no frequency drift during their courses. 
The salient pitch of many samples of recorded 
background apparently changed progressively 
during the course of the reverberation decay. 
Such samples were not used in the masking 
tests. Measurements on the film and aural 
matching of the pitch of echoes and reverbera- 
tions to that of an oscillator tone of known fre- 
quency showed that in no case did the frequen- 
cies of the echoes and the reverberations which 
were finally selected for test purposes differ 
from each other by more than 5 cycles. 

Reverberation records were also selected for 
absence of marked echo-like blobs as well as 
transmissions from other echo-ranging vessels. 
It was found that such “clean” reverberations 
gave more consistent results, with fewer false 
echoes reported. This careful selection of rever- 
beration samples is obviously not matched 
under field conditions. The results consequently 
apply to the ability of an operator to hear 
echoes in the presence of masking reverbera- 


UCDWR TESTS 


219 


tion. The problem of distinguishing between 
echoes from submarine and nonsubmarine tar- 
gets is a separate one and has received separate 
study.® 

The reverberation playbacks, for the longer 
transmissions, were ringing and tonal in char- 
acter, and fluctuated markedly in loudness dur- 
ing the period of their general decay. Frequent- 
ly, these fluctuations, or blobs, sounded very 
much like echoes, and in preliminary tests were 
commonly mistaken for echoes. For shorter 
transmissions, the ringing character became 
progressively less noticeable and the sound ac- 
quired a noisier quality. The loss of tonality is 
greatest for reverberations produced by pings 
less than 10 milliseconds in duration, and the 
sound in this case more nearly resembled that 
of a reverberant gunshot. A narrow-band 
analysis made of a 100-millisecond CW rever- 
beration with the aid of a 5-cycle filter indi- 
cated a total frequency spread of about 20 
cycles, in agreement with the 2/t criterion dis- 
cussed in Chapter 7. 

Fluctuations of intensity are still perceptible 
in reverberations from short transmissions but 
are less striking because of the small duration 
of the blobs and the consequent fact that the 
ear tends to integrate over a greater number 
of the shorter blobs. In other words, the 
perceived background is smoothed to some ex- 
tent because of the ear’s build-up time. In the 
reverberation samples obtained with trans- 
missions 114 and 271 milliseconds long, there 
seemed to be less correlation between the 
lengths of the blobs and the lengths of the 
emitted pulses than previous studies^ had indi- 
cated for the shorter transmissions. 

In all but a few of the tests the sounds were 
presented through high-quality headphones 
worn by the observers, who could locally vary 
the total gain over a range of about 15 decibels 
to obtain the level of greatest comfort. The 
initial blast of the reverberation set an upper 
limit to the value of the gain which could be 
used for long listening periods; and, in the 
fixed-range tests described below, the echo was 
injected at a point in the reverberation decay 
characteristic corresponding to a loudness level 
of approximately 50 phons. In general, the 
level of the reverberation background at the 
instant that the echo was injected produced at 


least 20 decibels more masking than did room 
noise. Whenever excessive amounts of room 
noise occurred during a test, the results were 
rejected and a retest given. Except in the tests 
in which the effect of distortion was studied, 
the total amount of nonlinearity in the record- 
ing and reproducing systems did not exceed 1 
to 3 per cent. This precaution was necessary in 
order to avoid introduction of spurious pitch 
differences. 

In order to reproduce the echoes and rever- 
berations with the same pitch, the motions of 
the film loops in the signal and background 
channels were synchronized. This procedure is 
important for the additional reason that it 
permits injection of the echo at the same range 
in each presentation and thereby simplifies the 
task of correlating echo detectability with the 
time-amplitude pattern of the reverberation 
background. Despite all efforts to obtain abso- 
lute synchronism, very slight short-term fluctua- 
tions of film speed remained, so that there were 
residual variations in the phase relations be- 
tween echo and reverberation in successive pres- 
entations of a given echo against a given 



Figure 8. Unrectified CRO traces of echo-rever- 
beration mixtures for a high signal-to-background 
ratio. The arrows point to the echoes. 


reverberation at a “fixed” range. In Figures 
8 and 9, for example, are reproduced oscillo- 
grams showing the time-amplitude patterns of 
successive mixtures of a given echo without 
doppler and a given reverberation at a fixed 
range, and also traces of the rectified, smoothed 
signal obtained from the echo-reverberation 
mixture. The only difference between succes- 
sive traces is the random variation in the level 
of the echo-reverberation mixture. In the first 
of these figures, the echo-to-reverberation ratio 
exceeds the value corresponding to primaudi- 
bility, and the two sounds are of different 
amplitudes. The total variation in level of the 
mixture amounted to about 2 decibels in this 
case. In Figure 9, the echo-to-reverberation 
ratio was very nearly that corresponding to 
primaudibility. Since, in this case, echo and 



220 


REVERBERATION MASKING OF SIGNALS WITHOUT DOPPLER 


reverberation were of nearly equal amplitudes, 
the phase interference effect was large, amount- 
ing to about 7.6 decibels, which appears to be 
nearly the maximum range of variation to be 
expected from this effect. The amount of the 
displacement of the two film loops relative to 
each other as they pass the light gate, which 
is required to produce a phase shift of 180 
degrees, is only 0.01 inch for a sound frequency 
of 800 cycles. This appears to be the irreducible 
minimum in the variation from absolute syn- 
chronism which is inherent in the equipment 
used. 


differentials for the case of the 271-millisecond 
signal (see Figure 12) . 

Measurement of Signal and Reverberation 
Levels 

The levels of echoes and reverberations were 
found by measuring the deflections which these 
sounds produced on the screen of a cathode-ray 
oscilloscope. In practice, the screen was photo- 
graphed on a continuously moving film. Since 
the oscilloscope had a linear scale, the measured 
deflection on this special film was then propor- 
tional to the echo or reverberation amplitude 




Figure 9. Unrectified and rectified CRO traces of echo-reverberation mixtures for primaudible signal-to- 
background ratio. The arrows point to the echoes. There is no correspondence between traces in the upper 
and lower sets. 


The major result of this phase interference 
effect is apparently to change the level and 
envelope of the presented signal, although some 
distortion of the effective echo envelope may 
also be produced. In practice, such interference 
effects will occur before rather than after 
heterodyning, but it does not seem likely that 
reversal of the order of these operations will 
significantly affect the results. As will be clear 
from the results reported in the following sec- 
tion, echo and reverberation are not likely to 
be of approximately equal amplitudes at prim- 
audibility except in the case of the longest 
echoes. This factor may have contributed some- 
what to the scatter among observed recognition 


at any time. This method was found preferable 
to making measurements on the original sound 
track, which was too unwieldy and which had 
to be protected from excessive handling. Oscil- 
loscope records made in this way are repro- 
duced in Figure 7 in this chapter and also in 
Figure 9 of Chapter 8. 

Records such as these were used for measur- 
ing the duration and amplitude of the different 
echoes used. Most of the echo amplitude 
measurements were made at the peak levels of 
the echoes. For the single-point measurements 
(see Section 9.2.2), the levels at echo midpoint 
were used in computing recognition differen- 
tials. It was found for these beam-aspect echoes 





UCDWR TESTS 


221 


that the average echo level never differed from 
the midpoint level by more than 1 to 2 decibels. 

In measuring the reverberation amplitude by 
this method, a time scale was required to select 
the proper point for measurement. This was 
provided by means of a 100-cycle tone which 
actuated at 10-millisecond intervals a neon 
lamp mounted at the edge of the screen. By 
photographing the screen and the successive 
flashes of the neon lamp, a film record was ob- 
tained of the instantaneous reverberation am- 
plitude together with the time after reverbera- 
tion onset, or range (see Figure 7). 

Test Procedures 

In any test, the echo level was varied ran- 
domly over a range of 14 decibels containing 
7 steps of 2 decibels, and about Ys of the 
presentations were blanks. The onsets of suc- 
cessive reverberations were spaced 3 seconds 
apart to simulate a 3-second keying interval, 
which was found to allow the observers sufficient 
time to record their judgments of echo audi- 
bility. In this manner, signal-to-reverberation 
ratios could be varied from nearly certain to 
vanishing detection probability. Since it was 
found that some sequences of signal-to-rever- 
beration ratios tend to produce inversions of 
performance, two successive echo levels were 
never permitted to differ by more than 6 deci- 
bels. For example, weak echoes are often 
missed when they are presented immediately 
following very strong ones, although subse- 
quent weak echoes of the same magnitude as 
the first would generally be reported as audible. 
Similarly, when an audible but weak echo fol- 
lows a long succession of no-echo intervals, 
there is a tendency to miss such a weak echo. 
On the other hand, a blank following a succes- 
sion of strong echoes is not usually likely to 
lead to a commissive error unless the rever- 
beration presented during the blank interval 
contains an echolike blob. Although it is im- 
portant to be alert to the influence of effects 
such as these in fundamental laboratory stud- 
ies, in order to improve the stability of the 
test results, it is clear that such favorable con- 
ditions cannot generally be expected in the 
field. Hence, it may be valuable in further 
studies to obtain quantitative estimates of the 
probable magnitudes of such effects in deter- 


mining detection probability under service con- 
ditions. 

Ordinarily the total duration of each test 
was about 20 minutes. This length of time 
is comparable to a half-hour watch and was 
found to be short enough so that fatigue ef- 
fects were unimportant. While no controlled 
study was made of the influence of fatigue, 
it was found in a number of tests which were 
longer than usual that fatigue may have a 
large and unfavorable influence on the stabil- 
ity and excellence of performance. 

It was pointed out earlier that the reverbera- 
tions used in these tests were carefully se- 
lected in order to eliminate spurious effects, 
such as differences in pitch between echo and 
reverberation, and progressive drifts of rever- 
beration frequency with time. Hence, prelim- 
inary tests were conducted in order to deter- 
mine the minimum number of reverberations 
which could be used (for testing with an echo 
of given duration) without introducing signifi- 
cant changes in performance produced by mem- 
orizing the reverberation samples. It was 
found that, in general, 5 reverberation samples 
was a sufficiently large number so that memory 
effects became relatively unimportant. Thus, 
the BD observed for a particular echo, in- 
jected at fixed range, when the reverberation 
film loop contained 11 reverberations was only 
1 decibel worse than for the same echo in- 
jected at the same range when the reverbera- 
tion film loop contained only 5 of the 11 sam- 
ples previously tested. When the background 
loop contained only 1 reverberation sample, 
repeated over and over again, the improve- 
ment in BD as compared with an 11-sample 
loop amounted to between 1 and 4 decibels, 
depending on the pulse length. Memorizing 
was easier for the longer transmissions be- 
cause the blobs in the resultant reverberation 
were longer and therefore fewer in number. 

Except when otherwise stated, all UCDWR 
tests were made with a background loop con- 
taining 5 reverberation samples. No explicit 
correction, which would amount to approxi- 
mately 1 decibel, has been applied to these 
data in order to allow for the effects of mem- 
orizing the background samples, and this 
should be borne in mind when assessing the 
test results. During these tests on memory 


RESTRTCTEB 


3 


222 


REVERBERATION MASKING OF SIGNALS WITHOUT DOPPLER 


effects, it was found that often a very weak 
echo could be detected because its injection 
produced a slight but detectable change in 
the time-amplitude pattern of the reverbera- 
tion. None of the results quoted here reflect 
the influence of this factor since it is, in gen- 
eral, quite unrealistic and all data influenced 
by it were eliminated from the final results. 
However, bottom reverberation received in 
anchored echo-ranging gear, such as is used 
in the protection of harbor entrances, may 
have a structure which tends to recur in suc- 
cessive reverberations. In such cases, recollec- 
tion of reverberation envelope might play a 
significant role in submarine detection. 

The echoes used in most of these tests were 
injected at fixed range in order to simplify 
the study of correlation between audibility and 
reverberation envelope; it was this aspect of 
test procedure which necessitated caution in 
minimizing the effects of memory. One lis- 
tener who participated in a small number of 
tests had a highly developed sense of rhythm 
and could tell from the rate of succession of 
the blobs the precise instant at which to con- 
centrate on the echo in the case of the fixed 
range tests. However, as pointed out above, 
there were usually so many variations in re- 
verberation envelope and they followed each 
other so rapidly that mere knowledge of the 
approximate range was not a very helpful cue 
to the average observer when the number of 
reverberation samples exceeded 5. This is con- 
firmed by the fact that recognition differentials 
obtained in tests in which the range was varied 
are in good agreement with the fixed range 
recognition differentials. 

The average incidence of commissive errors 
occurring in all tests in this program was 3.5 
per cent. The greatest incidence was noticed 
in the 11-millisecond tests, where the relative 
number of such errors rose to 7 per cent. How- 
ever, even for the 11-millisecond echo no more 
than 1.5 per cent of commissive errors were 
made when a different and “clean'er” set of 
reverberations was substituted. The tendency 
toward commissive errors was minimized by 
interviewing and retesting observers whose 
votes showed a bias toward multiple commis- 
sive errors. 


Training Effects 

The observers used in these tests were labo- 
ratory personnel, many of whom had previ- 
ously participated in tests in which the mask- 
ing of target sounds and of recorded echoes 
by noise background were studied. Generally, 
the tests were administered to groups of five 
observers. In standard psychological tests of 
auditory acuity, the Seashore tests, the group 
scores were found to fall in the highest decile 
for pitch discrimination and in the second 
decile for loudness discrimination. One mem- 
ber of the test group possessed an accurate 
sense of absolute pitch. Examination of abso- 
lute auditory thresholds showed no significant 
hearing loss up to 10 kilocycles for any mem- 
ber of the test group. Comparison of recogni- 
tion differentials obtained in one series of re- 
verberation masking tests, in which the regular 
test group and also five sonar instructors at- 
tached to the West Coast Sound School partici- 
pated, showed that the performance of both 
groups of listeners was the same to within 1 
decibel, and these small differences showed no 
systematic trend within the test series on which 
the comparison was based. 

It should be pointed out, however, that in 
effect both of these groups consisted of experi- 
enced listeners. The UCDWR observers were 
given a thorough preliminary orientation in 
the nature of the sounds to be used in the 
tests, since it was noticed early in the test pro- 
gram that failure to supply such preliminary 
experience often resulted in failure to report 
fairly strong echoes, especially for short ranges 
and pulse lengths, or produced consistent er- 
rors of commission. This phase of listening in- 
doctrination produces an understanding of the 
problem. It involves a fairly abrupt change in 
listening performance, and, once understand- 
ing exists, performance is much more stable. 
This kind of indoctrination is obviously impor- 
tant in training sonar operators. 

During the course of the listening tests, the 
performance of observers also showed a ten- 
dency to improve slowly and progressively with 
increasing experience. This effect is rather 
important in listening studies involving elusive 
and subtle distinctions between echoes and 
blobs. Thus, when the group of listeners was 


UCDWR TESTS 


223 


shifted from tests in which beam-aspect echoes 
were reverberation-masked to tests in which 
similar echoes were noise-masked, and then 
after a period of a week or two were returned 
to reverberation tests, it was found that their 
performance in the reverberation tests had de- 
teriorated by about 3 decibels from the previ- 
ous standard. However, after a week or more 
of reverberation testing, the group perform- 
ance in the reverberation tests had returned 
to its earlier value. This observation implies 
that periodic field exercises, refresher courses, 
or the opportunity to listen to recordings of 
typical echo-reverberation mixtures would be 
valuable in maintaining a high standard of 
performance among sonar operators. 

Since all the results reported were obtained 
under conditions in which the effect of ex- 
perience was relatively constant, comparisons 
between them do not reflect the influence of 
this factor to any significant extent. However, 
it should be pointed out that there is an irre- 
ducible minimum of variability in performance 
arising from the nature of the material selected 
for test purposes. Thus, it was observed in 
these tests that two different echoes of the 
same length and general level, but with some- 
what different envelope shapes, were not 
equally audible in the presence of reverbera- 
tion. Similarly, the variability in performance 
for this group of listeners was considerably 
less in the reverberation masking tests than 
in the noise masking tests reported in Section 
8.4. This is presumably due in part to the 
fact that the noise tests were preliminary in 
nature and the total testing period too brief 
to permit the listeners to acquire enough ex- 
perience to give very stable performance. 

It is interesting to note in this connection 
that one of the effects of training is to make 
the responses more nearly automatic. Thus, 
it was found that experienced observers usually 
would do just as well regardless of whether or 
not they made an active effort to concentrate 
on each item; in other words, rather passive 
listening could apparently be tolerated without 
significantly affecting performance. This find- 
ing implies that the results obtained in these 
laboratory tests may be applicable to the field 
situation despite the greater number and vari- 
ety of distractions encountered in practice. 


Observed Recognition Differentials 
Fixed Range Tests 

Recognition probability curves for the five 
echo lengths studied in these tests are shown 
in Figure 10. In this case, the echo-to-rever- 
beration ratio shown on the horizontal scale is 



ECHO-TO-REVERBERATION RATIO IN DB 


Figure 10. Probability of detecting an echo 
masked by reverberation. Each curve is an average 
of five transition curves obtained with an echo of 
the indicated duration in milliseconds. 

defined in terms of reverberation and echo lev- 
els measured at the midpoint of the echo. These 
curves are composites, showing the mean per- 
formance of groups of five observers in tests 
in which the echo was injected at three dif- 
ferent ranges varying from about 900 to about 
2,000 yards. For each of these three ranges, 
15 separate determinations of recognition 
probability were made. For each pulse length, 
however, only a single echo was used. Failure 
of these curves to approach zero as rapidly as 
they approach 100 per cent recognition proba- 
bility is due to the influence of commissive er- 
rors, although as shown by the curves, and as 
pointed out above, the effects of guessing were 
relatively small in these tests. Transition 
curves given by observers who try to force 
their thresholds down tend to be much more 
gradual in slope than are the curves shown 
in Figure 10, and such transition curves often 
show inversion at the lower signal-to-back- 
ground ratios (see Figure 7D in Chapter 4). 
The mean value of n for these transition curves 
is about 1.8. 

The data on which the transition curves 
shown in Figure 10 are based have been plot- 


RESTRICTED I 


224 


REVERBERATION MASKING OF SIGNALS WITHOUT DOPPLER 


ted in a slightly different way in Figure 11 in 
order to show the fractional part of the test 
group for which the echo was audible in at 



Figure 11. Fraction of test group detecting echoes 
in at least 50 per cent of trials for echoes of the 
indicated durations in milliseconds. 

least 50 per cent of the trials at various values 
of the signal-to-reverberation ratio. It will be 
seen that the difference in performance of the 
best and poorest observers did not exceed 6 to 
7 decibels and that this difference did not de- 
pend strongly upon pulse length. Nearly iden- 
tical results were obtained when the quantity 
plotted represented the fraction of the test 
group which could detect at least 20 per cent 
and also at least 80 per cent of the echoes at 
various signal-to-reverberation ratios. 

The 50 per cent recognition differentials 
found by averaging the individual recognition 
differentials from each transition curve are 
shown as filled-in circles in Figure 12. Each 
point represents an average of 45 determina- 
tions (15 tests at each of 3 ranges) made, how- 
ever, with a single recorded echo. The length 
of the vertical lines drawn through these cir- 
cles represents the rms deviation from the 
mean of the results which occurred in the in- 
dividual tests. The numerical values of these 
rms deviations are listed in Table 2 and are 
discussed more fully at that point. These rms 
deviations represent the total effect due to all 
sources of variation. 


The inversion in recognition differentials for 
the 97-millisecond and 114-millisecond echoes 
was checked and confirmed in an additional 
series of tests. This anomaly may be associated 
with the envelope of the 114-millisecond echo. 
In fact, for one of the 114-millisecond echoes 
studied in the retest series, the RD was con- 
siderably lower than for the 114-millisecond 
echo used in the main body of this work, al- 
though the same set of reverberation samples 
was used in both cases. It may also be noted 
that the 11- and 97-millisecond echoes and re- 
verberations were recorded on a different day 
from that of the other sounds. Consequently 
sea conditions may have affected reverberation 
character for the 11- and 97-millisecond tests, 
lowering the RD and increasing the rms devia- 
tion for both of these cases with respect to the 
other three shown in Figure 12. 

The continuous line in Figure 12 has been 
drawn as a curve of best fit for the filled-in 



ECHO LENGTH IN MILLISECONDS 

Figure 12. Aural detection of CW pulses and 
echoes at 800 to 1,000 cycles, masked by reverbera- 
tion from transmitted CW pulses equal in length 
to the signal length and heterodyned to the same 
audio frequency. The filled-in circles apply to 800- 
cycle beam echoes, for which the recognition differ- 
ential is defined by measuring the echo and rever- 
beration at echo midpoint ; the vertical lines repre- 
sent standard deviations among the recognition 
differentials for specific echoes. The open circles 
apply to filtered 800-cycle pulses, for which the 
recognition differential is defined by measuring the 
average level of a group of reverberation samples 
at echo midpoint by means of a power level 
recorder. The open squares apply to 1-kilocycle in- 
jected pulses, for which the recognition differential 
is defined by measuring the levels of neighboring 
reverberation peaks. 

circles representing the UCDWR results. The 
dotted portion of this curve has been drawn 
on the basis of some very preliminary data ob- 


estricted| 



UCDWR TESTS 


225 


tained at UCDWR with signals less than 10 
milliseconds long. These preliminary tests in- 
dicate that the audibility of short signals 
masked by reverberation deteriorates much less 
rapidly with diminishing pulse length than does 
the audibility of longer echoes. 

For comparison with the continuous line in 
Figure 12 the open symbols represent recogni- 
tion differentials obtained in other programs. 
In particular, the open circles represent the re- 
sults obtained by BTL described in Section 
9.1.2, while the open squares give the recog- 
nition differentials obtained by the British 
(see Section 10.2.3). The agreement between 
these four points and the curve based on the 
UCDWR data is not very impressive, but all 
the points lie within 3 decibels of the curve 
shown in the figure. The British data exhibit 
the same change with pulse length as do the 
UCDWR results. Since the absolute values of 
the recognition differentials obtained by the 
British are somewhat uncertain, this agree- 
ment is as close as could be expected. The BTL 
results, on the other hand, show a change of 
9 decibels as the pulse length decreases from 
100 to 25 milliseconds, and this change is much 
greater than can be reconciled with the 
UCDWR results. Since the latter extend over 
a greater range of pulse length, they are be- 
lieved to be more reliable. The discrepancy 
is puzzling, however, and casts some uncer- 
tainty on the reliability of the curve of best 
fit shown in Figure 12. Possibly the fact that 
the 140-cycle filter used in the BTL tests was 
centered 35 cycles away from the midfrequency 
of pulse and reverberation may have impaired 
recognition of the 25-millisecond pulse relative 
to the 100-millisecond pulse. 

The continuous curve in Figure 12 may be 
used to estimate the advantage to be expected 
by reducing the pulse length when reverbera- 
tion is the dominant element of background. 
As stated in Section 7.3, the average intensity 
of reverberation background at a given range 
(for fixed oceanographic conditions and power 
output) is inversely proportional to pulse dura- 
tion. In other words, reverberation intensity 
is proportional to the energy in the emitted 
pulse ; hence, a tenfold reduction of pulse length 
results in a 10-decibel drop of reverberation 
level for a specific set of conditions. Thus, the 


fact that the recognition differentials shown 
in Figure 12 deteriorate at a rate less than 
10 decibels per tenfold decrease of pulse length 
means that weaker echoes can be detected in 
the presence of reverberation, everything else 
being the same, when the pulse length is short- 
ened. Figure 13 gives an estimate of the 



Figure 13. Estimated improvement in echo de- 
tectability resulting from decreased pulse length, 
when reverberation is limiting. 


amount of this improvement and is based on 
the probable recognition differentials shown by 
the continuous line in Figure 12. According 
to Figure 13 there is an estimated improve- 
ment of about 5 decibels in changing from a 
100-millisecond to a 10-millisecond transmis- 
sion. This estimate is in agreement with the 
practical observation^ that shortening the 
transmission is helpful in the presence of re- 
verberation. However, loss of ability to dis- 
tinguish doppler and thus to discriminate wake 
from target echoes makes a short transmission 
undesirable for search when the echo is to be 
detected by ear, although shortening the pulse 
may be helpful during action and is probably 
somewhat more helpful when nonauditory de- 
tectors are used. It should be noted that these 
results are valid only for clean echoes and are 
thus applicable only to submarines at beam 
aspect. At other aspects, smear echoes appear, 
the echo becomes prolonged, and in addition its 
mean intensity decreases proportionally with 
the pulse length. The relative advantage for 
short pulses shown in Figure 13 is, therefore, 
not generally applicable. 

It is interesting to compare the recognition 
differentials found for reverberation-masked 
echoes with those given in Chapter 8 for noise- 
masked pulses. This comparison may be ex- 
pected to cast light on the fundamental proc- 



226 


REVERBERATION MASKING OF SIGNALS WITHOUT DOPPLER 


esses involved in auditory detection of pulses. 
One important difference must be noted before 
this comparison can be made. When a pulse is 
masked by reverberation, the measured signal- 
to-reverberation ratio gives directly the ratio 
of signal to reverberation in each of the ear’s 
critical bands, which are stimulated, since the 
spectrum of signal and reverberation are al- 
most identical, unless own-doppler broadens the 
reverberation spectrum by a large amount (see 
Section 10.1.2). With a noise background, on 
the other hand, the spectra of signal and back- 
ground are quite dissimilar, and an assumption 
must be made to determine the signal-to-back- 
ground ratio in a critical band. Specifically, the 
equivalent filter width of a critical band must 
be known so that the intensities of signal and 
background in this band can be computed. In 
Section 8.4.4 the signal-to-background ratio of 
the primaudible pulse in one of the ear’s critical 
bands has been computed and the result plotted 
in Figure 11 of Chapter 8. 

This computed curve showing the ratio of 
primaudible signal to noise in one of the ear’s 
critical bands has been transcribed to Figure 
14 where it is shown as the upper dashed 



Figure 14. Comparison of recognition differen- 
tials for 800-cycle signals masked by noise and re- 
verberation. Curve A shows critical-band recogni- 
tion differentials for wide-band noise backgrounds 
(Figure 11 in Chapter 8). Curve B shows recogni- 
tion differentials for reverberation measured by 
the single-point method (Figure 12), while curve 
C shows recognition differentials for reverberation 
measured with an electrical time constant of 100 
milliseconds (see Table 3). 


curve. The continuous curve for reverberation 
masking is also given, transcribed from Figure 
12. Since the two curves were prepared quite 


independently of each other, the parallelism 
shown is striking. This figure gives added sup- 
port to the inference, stated in Section 8.5.3, 
that the critical-band RD for a very brief pulse 
tends to be independent of pulse length, ap- 
proaching a constant value of about 14 deci- 
bels. This result is rather surprising and at 
present must be regarded as suggestive rather 
than conclusive. In particular, more data on 
reverberation masking of pulses about 1 milli- 
second long are required to test the reality of 
this apparent result. 

Curve C in Figure 14 represents the recogni- 
tion differentials found by electrical integration 
of the reverberation level, which are discussed 
below. The failure of this curve to agree with 
the curve B casts some doubt on the validity of 
either and suggests that the tentative agree- 
ment between the curves of RD for reverbera- 
tion and noise masking may not be real. How- 
ever, as pointed out below, it is difficult to 
understand why the electrical integration gives 
such systematically different results from those 
obtained by the single-point method. Evidently 
more data are required before definite conclu- 
sions can be drawn. 

The fact that the reverberation recognition 
differentials shown in Figure 14 are somewhat 
smaller than the critical-band recognition dif- 
ferentials obtained in the presence of wide- 
band noise may be accidental. If corrections 
were introduced for the effects of memory and 
for use of a decibel, or power level, rather than 
a power average in obtaining mean recognition 
dilferentials for reverberation masking, the lat- 
ter values (at least those taken from Figure 
10) would probably tend to lie somewhat above 
the critical-band recognition differentials for 
noise masking. Also, the loudness level per 
critical band was considerably less when wide- 
band noise was used than for the reverberation 
tests, since the overall backgrounds probably 
had much the same loudness in both sets of 
tests. Since recognition becomes more diffi- 
cult at lower loudness levels (see p. 231), 
this effect would account for an increase of 
noise-masking recognition differentials com- 
pared to those for reverberation masking. It 
would be expected that when all these factors 
were taken into account, the critical-band 
recognition differentials for reverberation 


UCDWR TESTS 


227 


should exceed those for noise, since misleading 
cues like reverberation blobs are much less 
likely to occur in the case of masking by wide- 
band noise. However, the RD at 10 millisec- 
onds found by electrical integration (see Table 
3) seems too far below the noise masking RD to 
be brought above it by any of these corrections. 

Variability 

The individual recognition differentials 
which were averaged to give the plotted points 
in Figure 12 showed a high degree of variabil- 
ity. The origin of this variability was the sub- 
ject of considerable study at UCDWR, and the 
results obtained cast some light on the basic 
mechanism producing echo masking. 

The nature of this variability was found to 
be intimately connected with the manner in 
which the reverberation intensity was mea- 
sured. For the data plotted in Figure 12, 
instantaneous levels of echo and reverberation 
were measured at a time corresponding to the 
midpoint of the echo. This method is referred 
to hereafter as the single-point method. As 
shown in Figure 12 and Table 2, the rms devia- 
tion among recognition differentials calculated 
by the single point method is of the order of 4 
decibels ; hence, the total range of variation is 
about 12 to 14 decibels, and extreme deviations 
from the mean may be as large as 6 to 8 
decibels. 

The arithmetic meaning of this scatter is 
best understood by referring to Figure 15. In 
this figure the level of the primaudible echo is 
plotted against the reverberation level mea- 
sured by the single-point method. The differ- 
ent points represent measurements made with 
a single 79-millisecond echo against eleven dif- 
ferent reverberations, at each of two ranges. 
It is clear from this figure that the reverbera- 
tion level measured by the single-point method 
seems to have only a general relationship to the 
level of the echo that can just be detected. 

If the primaudible echo level were equal to 
the reverberation level plus the recognition 
differential, the points in Figure 15 should lie 
close to a 45-degree straight line, deviations 
from the line resulting from inaccuracy of the 
measurements. Some curvature of the line 
would be expected if the RD varied with the 


loudness level, but for the loudnesses used this 
effect was probably negligible. Thus the 45- 
degree straight line shown in Figure 15 was 
drawn. It will be noted that this line intersects 
the vertical axis at about 10 decibels, within 2 



REVERBERATION LEVEL 
IN DB 

Figure 15. Recognition differentials computed by 
the single point method. The two sets of symbols 
apply to different echo injection ranges. Echo and 
reverberation levels are given in decibels above the 
same arbitrary reference level. 

decibels of the mean curve drawn in Figure 12 
from more complete data. This intercept is 
clearly the RD, since the latter is the difference 
between signal and background levels. 

The scatter from this straight line is so 
large, however, that doubt is cast on the mean- 
ing of recognition differentials found with the 
single-point method. All the obvious possible 
causes of such scatter are ruled out, however. 
The physical measurements of echo and rever- 
beration level, following the techniques de- 
scribed in Section 9.2.1, were quite objective, 
and were reproducible to within a small frac- 
tion of a decibel. Similarly, when different ob- 
servers were used or when a test was repeated 
with the same observer, the level found for the 
primaudible echo injected at a particular range 
into a particular reverberation was moderately 
reproducible. In fact, the standard deviation of 
the primaudible echo determined for a fixed 
reverberation at a fixed range varied between 
1.0 and 1.2 decibels. This average scatter of 1.1 
decibels represents the variability of the ear’s 
performance. As a result of this irreducible 
scatter, the recognition differential for a par- 


RSfRlUTED# 




228 


REVERBERATION MASKING OF SIGNALS WITHOUT DOPPLER 


ticular presentation was measured with a scat- 
ter of only 1 decibel, or of the standard 
deviation found when the echo was injected 
into different reverberations or at a different 
range in the same reverberation. 

The conclusion may be drawn that the single- 
point method does not adequately measure the 
masking power of reverberation. In view of 
the rapid variability of reverberation and the 
integrating property of the ear, this is not a 
surprising result. We would expect that the 
masking power of reverberation would be de- 
termined by the average reverberation level, 
taken over an interval of several hundred milli- 
seconds, that is, over an interval comparable 
with the build-up time of the ear. Therefore 
other methods were devised to measure the 
acoustically significant reverberation level in 
an attempt to reduce the variability of the mea- 
sured recognition differentials. 

First, the reverberation level was measured 
at 5 points equally spaced over the echo, at 
the beginning, the end, and three equally spaced 
points during the echo. The recognition differ- 
entials obtained by this method, together with 
the standard deviation of recognition differ- 
entials, is given in Table 2. For comparison, 
the standard deviation of reverberation inten- 
sities from the mean intensity at each range 
is given on the right-hand side of the table. 
All the corresponding data for the single-point 


levels and of the calculated recognition differ- 
entials. 

This same point is also illustrated in Figure 
16. Here the primaudible echo level is plotted 
against the reverberation level for each back- 


PRIMAUDIBLE ECHO LEVEL IN DB 



Figure 16. Recognition differentials computed by 
the 5-point method. The two sets of symbols apply 
to different echo injection ranges. Echo and rever- 
beration levels are given in decibels above the same 
arbitrary reference level. 

ground sample and at a given range with each 
of 11 different samples. The points correspond 
to the same echo levels as in Figure 15, but the 
reverberation levels were measured by averag- 
ing over the echo. It is evident that the aver- 


Table 2. Recognition differentials determined by point and band methods. 


Echo length in 
milliseconds 

Recognition differential in db 

Standard deviations in db 

Recognition differential 

Reverberation level 


1 point 

5 points 

1 point 

5 points 

1 point 

5 points 

11 

11.7 

11.8 

4.6 

3.5 

4.5 

3.6 

36 

10.9 

10.3 

3.4 

1.9 

3.3 

1.7 

97 

6.4 

7.2 

5.4 

2.9 

5.1 

2.4 

114 

7.0 

6.6 

3.1 

1.7 

2.7 

1.3 

271 

3.7 

5.1 

2.9 

2.1 

2.4 

1.2 


method are also given in this table. It is evi- 
dent that averaging the reverberation intensity 
over 5 points considerably reduces the standard 
deviation both of the individual reverberation 


age reverberation level found in this way is a 
better indication of the masking power of re- 
verberation than is the level at a single point. 
Various other methods of measuring the 



UCDWR TESTS 


229 


reverberation level were tried. Five points 
prior to the echo were averaged ; also a planime- 
ter was used to integrate over the reverberation 
intensity prior to and during the echo. In addi- 
tion, a power level recorder was used to smooth 
the reverberation level. Each of these sig- 
nificantly reduced the scatter of the measured 
recognition differentials. The greatest reduc- 
tion in scatter was obtained with what is in 
practice a very simple method. An electrical 
system was used to smooth the reverberation; 
thus, the emergent signal at any time repre- 
sented an integral over the reverberation in- 
tensity at times prior to multiplied at each 
time t by The resulting integral was 

divided by to give an average reverberation 
intensity at the time Such average intensi- 
ties were computed for various values of the 
time constant and the values at the echo 
midpoint were used in determining recognition 
differentials. The resultant values of the recog- 
nition differential are given in Table 3, for dif- 
ferent values of As in Table 2, the observed 
standard deviations are also given both for the 


milliseconds prior to echo midpoint. This result 
is, of course, generally consistent with expec- 
tation. 

The recognition differentials given in Table 
3 are not directly comparable with those for 
the single-point method shown in Table 2 and 
Figure 12. Since the mean reverberation level 
decreases rapidly with time, the single-point 
recognition differentials are affected by the 
high reverberation prior to the echo. If the 
primaudible echo level is to be predicted from 
the mean reverberation intensity at the range 
of the echo, the single-point or five-point recog- 
nition differentials are probably valid, provided 
that the reverberation is heard directly, with- 
out any TVG, A VC, or similar device to main- 
tain the reverberation level constant. On the 
other hand, if the reverberation level presented 
to the ear does not change systematically with 
time, the recognition differentials in Table 3 
are probably more realistic. It will be noted 
that these give an even greater relative advan- 
tage for the shorter pulse lengths than that 
shown in Figure 12. 


Table 3. Recognition differentials determined by electrical integration. Time constant h of circuit given in milliseconds. 


Echo 

Recognition differentials 

in db 



Standard deviations in 

db 



length 

in 

milli- 





Recognition differentials 

Reverberation levels 


seconds 

/o = 200 

/o = 100 

^0 = 50 

<0 = 5 

<o = 200 

<o = 100 

<0 = 50 

<0 = 5 

<o = 200 

<o = 100 

< 0=50 

<0 = 5 

11 

7.8 

9.7 

10.1 

10.5 

1.5 

1.4 

1.3 

2.7 

0.7 

0.9 

1.2 

2.7 

36 

6.7 

9.1 

9.9 

10.0 

0.9 

1.1 

1.4 

1.6 

0.6 

1.0 

1.5 

1.5 

97 

3.8 

5.9 

7.0 

8.1 

1.1 

1.2 

1.9 

3.8 

1.1 

1.1 

1.8 

3.6 

114 

4.3 

6.3 

6.7 

7.3 

1.1 

1.3 

1.8 

2.2 

0.8 

1.2 

1.6 

2.0 

271 

1.1 

3.5 

4.2 

4.8 

1.4 

1.5 

1.8 

1.9 

0.9 

1.0 

1.3 

1.3 


recognition differentials and for the reverbera- 
tion intensities at a fixed range. It is evident 
that the standard deviations in this table are 
much reduced from those in Table 2. Especially 
for time constants of 100 or 200 milliseconds, 
these standard deviations are so close to the 
limiting value of 1.1 decibels set by variability 
in the ear’s performance that no further reduc- 
tion in variability along these lines seems pos- 
sible. The conclusion may be drawn that the 
masking power of reverberation is measured 
primarily by an integral extending 100 to 200 


From a practical standpoint. Table 3 implies 
that the recognition differential for an echo in 
the presence of reverberation should always be 
expressed in terms of the average reverbera- 
tion taken over an interval 100 to 200 milli- 
seconds preceding the echo. Since this is not 
very convenient, the reverberation levels dur- 
ing the echo may be used instead, together with 
the recognition differentials shown in Figure 
12, but with some loss of accuracy. Especially 
when the reverberation falls off very sharply, 
use of the recognition differentials found in 



STRICTE 



230 


REVERBERATION MASKING OF SIGNALS WITHOUT DOPPLER 


terms of the reverberation level during the 
echo and shown in Figure 12 may lead to er- 
roneous conclusions, and this possibility should 
be kept in mind. 

It will be noted that the recognition differ- 
entials shown in Tables 2 and 3 display some 
puzzling discrepancies. In particular, in Table 
2 the RD found by the five-point method for 
a 271-millisecond echo is 1.4 decibels higher 
than the corresponding RD found by the single- 
point method. Values for the other echo lengths 
are in good mutual agreement. On the other 
hand, in Table 3 the recognition differentials 
for time constants of 100 and 200 milliseconds 
show a systematically different change with 
echo length than do the recognition differen- 
tials in Table 2. The points for the time con- 
stant of 100 milliseconds have been represented 
by curve C in Figure 14. The curve for the 
200-millisecond time constant would be paral- 
lel, but would lie about 2 decibels lower. These 
differences arise entirely from differences in 
reverberation levels when averaged in different 
ways. There is no known reason why averag- 
ing over a fixed time interval should give lower 
values for shorter pulse length reverberation 
than for reverberation from longer pulses. 
Systematic differences between the reverbera- 
tions used might explain these discrepancies, 
although direct measurement seemed to indi- 
cate no systematic differences greater than 1 
decibel between the reverberation level at echo 
midpoint and the level 50 milliseconds previous. 
Some systematic error may affect all these mea- 
surements made with the electrical filters. It 
may also be noted that the recognition dif- 
ferentials obtained by use of the filter do not 
agree with the critical-band recognition dif- 
ferentials for noise, shown by curve C in Fig- 
ure 14. More data are required to clarify these 
differences between the recognition differen- 
tials determined in different ways. 

Measurements of recognition differentials 
were also made using a power level recorder 
to define the reverberation level at echo mid- 
point. The recognition differentials found in 
this way are given in Table 4. Since the method 
of reverberation measurement used was closely 
similar to that employed in the BTL tests and 
described in Section 9.1.1, these results are in- 
cluded here as a check on the validity of re- 


verberation measurements made with the 
power level recorder. It is evident from Table 
4 that the recognition differentials found are 
in substantial agreement with those obtained 


Table 4. Recognition differentials determined with high- 
speed power level recorder. 


Echo length 
in 

milliseconds 

Recognition 
differentials 
in db 

Standard de\ 

Recognition 

differential 

nations in db 

Reverberation 

level 

11 

13.0 

1.5 

0.6 

36 

8.5 

0.8 

1.1 

97 

7.8 

2.3 

1.8 

114 

6.2 

1.5 

0.7 

271 

4.0 

1.3 

0.9 


by use of the electrical filter, with a time con- 
stant of 100 milliseconds (see Table 3 and Fig- 
ure 14). Thus the high RD found at 25 milli- 
seconds in the BTL tests probably cannot be 
explained as a result of the method of rever- 
beration measurement used. 

It should also be mentioned at this point that 
the slopes of the composite transition curves 
based on single-point measurements and shown 
in Figure 10 are in general very nearly the 
same as those of the transition curves com- 
puted by segregating the echo recognition prob- 
abilities and plotting individual transition 
curves for each of the reverberations on a 
given test loop or for different ranges of the 
same reverberation. In other words, the abso- 
lute values of echo-to-reverberation ratios lead- 
ing to a particular detection probability de- 
pend on the method of measuring reverbera- 
tion level, but the rate of change of detection 
probability with increasing signal level is much 
the same for all reverberations and all ranges. 

The significance of the preceding discussion 
may be reviewed with the aid of the standard 
deviations of the measured reverberation lev- 
els also shown in Tables 2, 3, and 4. From 
these, it will be noted that, when the levels of 
the various reverberations are measured with 
an instrument having an integration time ap- 
proximately that of the ear, the scatter among 
the measured levels for different reverbera- 
tions at a given range is much reduced. Simi- 
larly, recognition differentials computed on the 


UCDWR TESTS 


231 


basis of integrated levels show about the mini- 
mum degree of scatter resulting from variable 
performance of the ear when presented with 
a constant physical situation. For practical 
purposes, the probable performance of the ear 
can best be predicted with the aid of an aver- 
age of single-point reverberation levels mea- 
sured at echo midpoint or by using integrated 
levels measured at echo midpoint, and there- 
fore indicating the level a short time in ad- 
vance of echo midpoint. These two sets of rec- 
ognition differentials may differ by several deci- 
bels because of the gradual decay of average 
reverberation level with time. 

Finally, it should be mentioned that the lev- 
els of successive echoes received at sea under 
apparently identical conditions may vary 
widely from the mean level of a group of such 
echoes. It follows that, in order to predict the 
detection probability for a particular echo, it 
is necessary to combine the echo distribution 
function, which describes the probability that 
an echo will have a given level under speci- 
fied conditions, with the transition curve speci- 
fying the probability that an echo of given in- 
tensity will be heard in the presence of a par- 
ticular noise or reverberation background.^ 

Initial Contact Experiments 

These tests were administered just prior to 
the fixed-range tests described previously. The 
same echo and reverberation loops were used in 
both types of test, the major difference being 
that the listeners did not know the injection 
range in the initial contact studies. In the 
initial contact work, successive echoes were 
injected at increasingly higher levels, begin- 
ning with levels definitely below primaudibility 
and ending with the level designated as well 
above threshold. To control guessing, blank 
intervals were included in the series of presen- 
tations. As in other ascending series minimal 
increment tests, the minimal increment RD was 
found to be in general 1 to 2 decibels higher 
than the random-order RD. However, the dif- 
ferences between the two types of test did not 
exceed the standard deviation of differences in 
performance in successive random-order tests 
and cannot be regarded as highly significant. 
This is especially true for ranges at which the 


character of the reverberation is confusing be- 
cause of the resemblance of blobs to echoes. 

Fixed and Variable Range Tests 

In the variable-range tests, a loop containing 
a single reverberation was used, and the echo 
level and injection range were varied simul- 
taneously. Range was varied in 200- to 400- 
yard steps, and separate records were kept for 
the responses at each range. The mean recog- 
nition differentials derived were not signifi- 
cantly different from those obtained in the 
fixed-range tests. Since the influence of mem- 
ory was greater in these variable-range tests, 
owing to the use of a single reverberation, it 
may again be inferred that ignorance of the 
injection range may hurt detection by 1 to 2 
decibels. Again, it was found that the charac- 
ter of the reverberation was worse and the 
instability of performance greater at some 
ranges than at others. 

Effects of Listening Level 

The reverberation level was varied in steps 
of 10 decibels from a level 5 decibels above the 
threshold of hearing (as determined by room 
noise, rather than the ear’s absolute acuity) to 
about 75 decibels above that threshold. The 
highest level used probably corresponded to 
between 90 and 95 phons. In order to avoid 
acoustic shock from the initial high level when 
the echo was presented at long injection ranges 
and at high listening levels, the earlier portions 
of the background on the single reverberation 
sample used were blanked out by means of 
opaque tape; thus, a larger range of listening 
levels could be tolerated. This procedure could 
not be employed in the variable-range tests, 
and, as Table 5 and Figure 17 show, use of 
level stabilizers may help by 2 to 3 decibels in 
practice by making it possible to use optimal 
gain for all ranges. 

The loudness level study was made with a 
114-millisecond echo only, and five observers 
were used. In this study, the echo level was 
increased progressively from a point well be- 
low threshold, and blank presentations were 
included. In other words, the curves in Figure 
17 were obtained essentially by what is called 


232 


REVERBERATION MASKING OF SIGNALS WITHOUT DOPPLER 


the increasing level method in Section 4.1.5. 

The effects of listening level are shown 
graphically in Figure 17. In general, 50 per 
cent recognition differentials improved pro- 
gressively with increasing level up to a sensa- 
tion level of about 50 decibels. The improve- 
ment (decrease) in recognition differentials 
with increasing level is listed in Table 5. 



Figure 17. Effect of gain setting on recognition 
probability of a 114-millisecond echo for different 
sensation levels. 


The performance curves for the last five 
levels in this table have been combined to give 
a single curve in Figure 17 because of the ex- 
tremely small differences shown in the indi- 
vidual curves. The general decrease in the 50 
per cent recognition differentials agrees with 


Table 5. Effect of loudness level. 


Listening level in db above 
lowest reverberation level 
audible in presence of room 
noise 

Decrease in recognition 
differential in db 

0 

0.0 

10 

0.6 

20 

1.7 

30 

2.6 

40 

2.8 

50 

2.6 

60 

2.9 

70 

2.7 


the trends shown in Chapter 4 (see Figures 6 
through 15 and Figure 65). The rms devia- 
tion between the performance of individual ob- 
servers and the mean of the group was about 
2 decibels. Hence, the trend shown applies to 
the performance of the group, rather than to 
that of individual listeners. In general, com- 
missive errors were comparatively few, but 
showed a tendency to increase with diminished 


listening level, so that the relative number of 
such errors was about three times larger for 
the lowest than for the highest listening levels. 

The influence of commissive errors, together 
with the greater difficulty of the assigned task 
(recognizing a single brief echo in the pres-, 
ence of a rapidly fluctuating background, as 
compared with recognizing a periodically re- 
peated variation in the level of a sustained 
sound) implies that the estimated dependence 
of transition curve slopes on listening level 
which is shown in Figure 65 of Chapter 4, 
is probably somewhat more reliable than the 
present results, at least for the case of recog- 
nition of target sounds. The total effect of 
loudness level on RD also appears smaller for 
the case of echoes than for that of sustained 
sounds and may be due to the reduced subjec- 
tive loudness of brief sounds. 

Perhaps the most significant inference from 
the loudness level tests is that RCG may prove 
helpful in practice. By this means a nearly 
optimal listening level may be maintained over 
the greater part of the reverberation limited 
range. Hence, residual masking effects due to 
the initially high level of received reverbera- 
tion may be minimized, operator fatigue re- 
duced, time of lost contact (due to the slowness 
with which manual gain adjustments are 
made) abbreviated, and the effects of over- 
loading and limiting which reduce the signal- 
to-reverberation ratio at very short ranges, if 
the gain is set at an average level, virtually 
eliminated. 

Effects of Distortion 

The types of distortion used were the same 
as those described in Section 8.4.7. With mod- 
erate distortion, the mean RD was reduced by 
0.9 decibel when the 565- to 1,130-cycle band- 
pass filter transmitted echo and reverberation. 
The standard deviation in successive tests was 
1.8 decibels. When the 0.1-9 kilocycle filter 
was used, the mean RD was increased by 0.5 
decibel, and the standard deviation was 0.8 
decibel. Thus, moderate distortion did not in- 
troduce any very significant change in the mea- 
sured recognition differentials. When the ex- 
treme type of distortion was used the RD was 
increased (caused to deteriorate) by 3.0 deci- 


restricte: 


UCDWR TESTS 


233 


bels and the standard deviation was 1.7 deci- 
bels. There can be little question that the last 
effect is statistically significant, in agreement 
with previous results. Again, the loudspeaker 


gave somewhat poorer results than the head- 
phones, and the difference between the two was 
greatest when signal and background were ad- 
mitted by the wider filter. 


RESTRICT ED 


Chapter 10 

REVERBERATION MASKING OF SIGNALS WITH DOPPLER 


S o FAR, attention has been centered upon the 
part played by the relative amplitudes of 
echo and reverberation in the process of de- 
tecting an echo without doppler. In the present 
chapter, the discussion is concerned primarily 
with the effects of pitch differences between the 
echo and reverberation produced by a CW 
transmission. Such pitch differences are pro- 
duced by motion of the target and/or the 
echo-ranging projector through the medium 
and are generally termed doppler effects. Dif- 
ferences between the pitch of echo and rever- 
beration which are produced by doppler are 
important because the masking effect of rever- 
beration is diminished thereby. In addition, 
doppler shifts enable the operator to distin- 
guish between stationary and moving reflectors 
and thus provide tactically valuable informa- 
tion. 


i DOPPLER EFFECT 

The equations specifying the magnitude of 
doppler shift are well known but are derived 
here for reference. Certain consequences of 
these equations are also discussed. Consider 
the situation in Figure 1, which represents a 



Figure 1. Reflection of sound from a target. 


sustained tone of wave length A emitted by a 
projector at the point 0. This wave is propa- 
gated through the medium in the direction OT 
and is reflected back from the target T to the 
receiver R. If the total distance of sound travel 
is L, and the lateral distance between projector 
and receiver is much smaller than L, then L 
equals 2r, where r is the range, or distance 
between projector and target. 


When the target and the projector-receiver 
are stationary, r is constant, and the phase 
angle <i>r of the sound received at R differs from 
the instantaneous phase </>o at the projector by a 
fixed quantity which depends on the number 
of wavelengths contained in the travel path L. 
The difference between these phase angles is 

0R — 00 = 27r ^ . (1) 

In other words, when L is an integral multiple 
of A, the receiver will lag the source by a multi- 
ple of 27r radians, and the two will move in 
phase. 

When the source and receiver remain sta- 
tionary and the target moves in a path perpen- 
dicular to the line OR, opening or closing the 
range at a velocity dr/dt, the difference be- 
tween the phase angles at source and receiver 
varies with time. This rate of change may be 
obtained by differentiating equation (1), which 
gives 


dt 


{<f)R — 0o) = — 


Itt dr , 

T dt 


(2) 


where mr and ojo are the angular velocities of 
the rotating vectors representing the instan- 
taneous phases of the received and emitted 
waves, respectively. In other words, wq equals 
27r/o where /o is the emitted frequency; simi- 
larly, o}R equals 27rfR, where /r is the received 
frequency. Since /o is fixed, equation (2) states 
that the frequency of the sound reflected by 
the moving target and received at R differs 
from that of the emitted sound by an amount 
depending on the rate dr/dt at which the range 
is opened or closed. Stated differently, motion 
of the target causes the rate of variation of 
phase and thereby the period of the received 
sound to differ from that of the emitted sound. 
When dr/dt is constant, A is independent of 
time and may be treated as a constant in dif- 
ferentiating equation (1). Also, when dr/dt is 
very small compared with c, the velocity of 
sound in the medium, A may be set equal to 



234 


DOPPLER EFFECT 


235 


either c/f^ or c/fn .without significantly affect- 
ing the numerical results obtained from 
equation (2). 

Substituting V for dr/dt, 27r/o and 27r//j for wo, 
and ix>R, c/fo for A, and rearranging reduces the 
previous equation to 

c 

where 8/ is the doppler shift /«— /q. The shift 
will be of positive sign — in other words, the 
received frequency will be higher than the 
emitted frequency — when the range is closing, 
and of negative sign when the range is opening. 
When 8/ is positive, the shift is termed '‘up- 
doppler”; when negative, “down-doppler."’ 

The restriction that the motion of the target 
be perpendicular to OR, that is, in the direction 
of the acoustic axis of the echo-ranging trans- 
ducer, is unnecessary. If the target T is actu- 
ally moving with the velocity V, as shown in 
Figure 2, at an angle 6 to the acoustic axis, then 



I 

ACOUSTIC AXIS 

Figure 2. Components of target velocity. 

the preceding argument applies to the com- 
ponent of motion along the acoustic axis, 
namely v, which equals V cos 0. Clearly, the 
component of motion parallel to the face of the 
transducer will not produce a doppler shift. 
Hence, equation (3) should be generalized to 
read 

Sf= ± V cos e. (4) 

Obviously, the preceding analysis applies 
equally well to the case of a stationary target 
and a moving transducer. Similarly, it applies 
when target and echo-ranging vessel are both 
in motion. Furthermore, it will be clear that 
the discussion leading to equation (4) requires 


only minor modification when the emitted 
sound is not a sustained tone but is instead a 
CW pulse of length r. In this case, the essential 
spectrum of the pulse consists of a group of 
components extending over a band A/, which 
is 2/t cycles wide, and each of these components 
will experience a frequency shift given by 
equation (4) . If the pulse is very short and its 
essential spectrum correspondingly broad, the 
essential spectra of the emitted pulse and the 
echo reflected from a moving target will over- 
lap each other unless the doppler shift 8/ is 
equal to the essential width A/ of the pulse 
spectrum. Thus, the minimum condition which 
must be met in order that the essential spectra 
of pulse and echo will not overlap is 

2 / / 2 /„\ 

-<(^-jFcos0, 

or 

^ To Vcose' 

When r is less than one-half the limit given in 
equation (5), resolution of the two spectra be- 
comes very difficult. 

Usually the frequency of the emitted pulse 
is in the supersonic region and for present pur- 
poses may be considered equal to 24 kilocycles. 
The received sounds are heterodyned to the 
region of audio frequencies. Since heterodyning 
merely subtracts a constant number of cycles 
from each component in the received sound, the 
magnitude of the doppler shift in cycles per 
second is the same in the heterodyned sounds 
presented to the ear as in the unheterodyned 
sounds in the water. Thus, use of a supersonic 
transmission frequency /o followed by hetero- 
dyning is advantageous because the doppler 
shift in the sounds presented to the ear is much 
greater than could be obtained by emitting a 
CW pulse at a low sonic frequency. 

Since most of the scatterers producing rever- 
beration may be considered as stationary, or 
nearly so, there are usually only two significant 
sources of doppler shift: motion of the echo- 
ranging vessel and motion of the target. The 
former shift, called own-doppler, is also given 
by equation (4) , if F is understood to represent 
the speed of the listening vessel and 6 the bear- 
ing of the transducer relative to the bow. Thus, 


restricted j| , 


236 


REVERBERATION MASKING OF SIGNALS WITH DOPPLER 


the center frequency of the reverberation spec- 
trum received when the transducer is trained 
forward will exceed that of the emitted pulse by 
2/oF/c; and the reverberation frequency will 
be less than the transmitted frequency by the 
same amount when the transducer is trained 
aft. In these cases, 6 equals zero or — tt, and 
cos 6 equals ±1. When the transducer is trained 
abeam, 0 equals 7r/2, and cos 6 vanishes. In this 
case the center frequency of the reverberation 
spectrum coincides with that of the emitted 
pulse. 


10.1.1 Magnitude of Doppler Shift 

It will be useful to evaluate orders of magni- 
tude for a number of doppler shifts. For ex- 
ample, the number of cycles of shift due to the 
radial velocity of a target opening or closing 
the range at a rate of 1 knot may be computed 
from equation (4). This calculation gives the 
pitch difference between target echo and rever- 
beration received on a given bearing. The re- 
verberation will in general have various 
amounts of own-doppler, but, unless the pitch 
of the reverberation is stabilized by some means 
such as own-doppler nullifier [ODN], the op- 
erator must judge the existence of target dop- 
pler by comparing the pitch of the echo and 
the reverberation. If we assume that the com- 
ponent of target motion V cos 0 in the direction 
of the transducer axis is 1 knot, or 1% feet per 
second, the doppler shift amounts to about 17 
cycles for an echo-ranging frequency of 24 kilo- 
cycles. The sound velocity c has been set equal 
to 4,800 feet per second in this computation. 
Thus the doppler shift per knot of target range 
rate is 17 cycles at 24 kilocycles. 

Since an echo-ranging vessel may often move 
at speeds of 20 knots or more, giving rise to 
positive and negative own-doppler shifts cor- 
respondingly larger than that computed here, 
the band width of an echo-ranging receiver 
operating at 24 kilocycles, and without ODN, 
should be at least 40 times 17 cycles, or 680 
cycles. Otherwise, the received frequency of an 
echo with a relatively small amount of doppler 
may lie outside the cutoff frequencies of the re- 
ceiver band, and may become so attenuated as 
to be inaudible. 


It should also be mentioned here that own- 
doppler in the reverberation produced by pings 
emitted from a moving antisubmarine vessel is 
quite marked when heard aboard a submerged 
submarine. The submarine sonar operator first 
hears the directly transmitted pulse. The 
emitted power is high, the distances of interest 
fairly small, and even when the projector is 
trained away from the submarine, the sound 
intensity is at least within about 30 decibels 
of its value on the axis; hence the directly 
transmitted pulse can always be heard aboard 
the submarine. The received intensity of this 
pulse will, of course, vary with the orientation 
of the search projector during the pinging 
cycle. Immediately following the ping, the sub- 
marine operator hears the associated reverbera- 
tion, produced largely by inhomogeneities 
toward which the projector of the surface ves- 
sel is oriented. If the projector is pointed 
toward the submarine, the reverberation will 
have the same pitch as the original pulse, ex- 
cept for frequency shifts resulting from the sub- 
marine’s motion through the water. When the 
projector is pointed away from the submarine, 
the direct signal, radiated from one of the 
minor lobes of the projector, has the same fre- 
quency as before, but the reverberation will be 
sound scattered out of the main sound beam 
over to the submarine and may have quite a dif- 
ferent frequency. 

Suppose, for example, that the submarine is 
on the surface vessel’s beam. The direct signal 
is therefore received at the frequency /o with- 
out doppler (except for that resulting from the 
submarine’s own motion). If the projector is 
pointed forward, the sound received by the 
water and scattered as reverberation will have 
a frequency greater than by the amount 
Vfjc, where V is the speed of the surface ves- 
sel in feet per second. When this sound reaches 
the submarine it still has this higher frequency. 
Similarly, when the projector is pointed aft, the 
reverberation heard aboard the submarine will 
have a lower pitch than the directly trans- 
mitted pulse. By correlating variations of pulse 
intensity with the doppler shift of the ensuing 
reverberation, it is in principle possible to ob- 
tain tactically useful information on the speed 
and course of the echo-ranging vessel, provided 
that the surface vessel carries out a regular 


DOPPLER EFFECT 


237 


search plan, sweeping over a fixed sweep sector 
at a uniform rate. 

101-2 of Reverberation Spectrum 

The effect of pulse length on the reverbera- 
tion spectrum has been discussed in Chapter 7. 
When the echo-ranging 'projector is in motion 
relative to the scatterers, the presence of own- 
doppler provides an added complication. Since 
own-doppler varies with the relative bearing 
and since the main lobe has an appreciable 
width, the reverberation received from differ- 
ent directions will have different doppler shifts. 
As a result, the spectrum of the received re- 
verberation will have added broadening. 

The magnitude of this effect may be com- 
puted without difficulty. Consider the rays in 
the main lobe which differ in direction from the 
acoustic axis by some angle A(9, measured in 
the horizontal plane. Let the resultant change 
of own-doppler from its axial value be denoted 
by A(8/). Since A(9 is a small quantity for stand- 
ard echo-ranging gear, A(8/) may be computed 
by means of a Taylor expansion of equation 
(4) in which only the first term need be con- 
sidered. This process yields 

m) = Ye = - Qt) ^ ® ® 

Hence, the spread in the reverberation spec- 
trum produced by own-doppler will be greatest 
when the projector has a beam orientation, 
and 9 equals 7r/2 or 37r/2. When the projector 
is trained forward or aft, and 9 equals 0 or — tt, 
A (8/) found from equation (6) vanishes. 
Higher order terms in the Taylor expansion 
give a nonzero value for A (8/) , but this value is 
generally so small that it may be neglected for 
most purposes. 

The spread in the reverberation spectrum 
given by equation (6) for a projector trained 
abeam is quite appreciable. If the width of the 
major lobe of the hydrophone is 6 degrees on 
each side of the axis, giving a value of about 
0.1 radian for A9, and if the speed is 20 knots, 
the width of the reverberation spectrum result- 
ing from finite beam width and own-doppler 
amounts to 70 cycles, since A (8/) found from 
equation (6) is about 35 cycles. This spread 
must be added to that resulting from the finite 


width of the emitted pulse spectrum. The cen- 
ter frequency of the reverberation spectrum re- 
ceived when the transducer is oriented toward 
90 or 270 degrees coincides, of course, with that 
of the center frequency of the emitted pulse 
spectrum. However, it may be noted that the 
echo received from a stationary target on which 
the transducer axis is not quite centered will 
show a doppler shift relative to the reverbera- 
tion. This shift also is produced by own-doppler 
and the finite width of the main lobe ; it is en- 
tirely independent of the existence of relative 
target velocity and makes difficult the determi- 
nation of target speed by means of doppler. 

It is clear that the spread in the reverbera- 
tion spectrum may depend on a variety of fac- 
tors other than that associated with the length 
T of the emitted pulse. Nevertheless, when r is 
10 milliseconds or less, the essential spectrum 
of the emitted pulse will be at least 200 cycles 
wide. Resolution of the spectra of reverbera- 
tion and echo will be difficult, unless target 
doppler is very large. Equation (5) indicates 
that for 6 knots of target motion, which is fairly 
high, amounting to a doppler shift of about 100 
cycles at an echo-ranging frequency of 24 kilo- 
cycles, the essential spectra of the echo and 
reverberation will overlap if the pulse length 
is less than 20 milliseconds. Resolution would 
probably be possible for somewhat shorter pulse 
lengths, but especially in view of the broad- 
ened reverberation spectrum produced by own- 
doppler, a pulse length of 10 milliseconds or 
less would make identification of target doppler 
difficult if not impossible. 

10.2 EFFECT ON RECOGNITION 

In a group of early British reports^"^ describ- 
ing tests under semifield conditions, evidence is 
presented on the effects which doppler may 
produce on echo recognition. From these data 
certain conclusions may also be drawn on the 
effects produced by changing the heterodyne 
frequency, the pulse length, and the injection 
range, and by substituting a loudspeaker for 
headphones. As indicated in earlier sections, 
precise definition of the various factors influ- 
encing test results is difficult even under labora- 
tory conditions. The present tests were intended 
merely to serve as guides to performance which 


RESTRICTEE^ 


238 


REVERBERATION MASKING OF SIGNALS WITH DOPPLER 


might be expected in the field with gear in use 
at the time the studies were made. While such 
an objective is extremely important opera- 
tionally, it is limited in that it often fails to 
establish whether the controllable factors which 
determine the level of performance have been 
properly exploited. 


10 . 2.1 Experimental Procedure 

A standard echo - ranging transducer, 
mounted on the bottom, as is customary with 
harbor-defense installations, was used to pro- 
duce the reverberations. Because of the shal- 
low depth of the water, these were essentially 
bottom reverberations. The transducer was op- 
erated at a frequency of 15 kilocycles, and the 
received reverberation was heterodyned down 
to an audio frequency of 1 kilocycle, which is 
standard British practice. In addition, 0.5 and 
1.5 kilocycles were also used as audio frequen- 
cies in some of the tests. The signal injected 
into the reverberation was produced by a 15- 
kilocycle oscillator provided with a calibrated 
vernier condenser by means of which the oscil- 
lator frequency could be varied from 14.9 to 

15.1 kilocycles in 10-cycle steps. Thus, the fre- 
quency of the audio pulse could be varied in 
either direction from that of the reverberation, 
and by amounts up to 100 cycles. The level of 
the injected pulse could be varied in steps of 
3 decibels; and the mixture of pulse and re- 
verberation was amplified and heterodyned to 
the audio region. 

Pulses of controlled duration were obtained 
from the oscillator by means of a magnetic 
relay which was actuated when a pair of mov- 
ing terminals made contact with a pair of sta- 
tionary terminals. In this manner, the dura- 
tion as well as the injection range of the CW 
pulse could be predetermined. Care was exer- 
cised to eliminate false cues produced by key 
clicks and transients. Internal evidence indi- 
cates that the envelope of the generated pulse 
was rounded rather than rectangular, so that 
spurious cues of the type discussed in Section 
9.1.2 played no significant role in the tests 
under discussion. With the aid of a CRO, the 
duration of the injected CW pulse was adjusted 
to equal that of the emitted pulse which pro- 


duced the reverberations. Pulse lengths of 10 
and 70 milliseconds were used. 

The amplifier used had a rather sharply 
tuned input circuit. Hence, its response, shown 
in Figure 3, produced a relative loss of approxi- 



Figure 3. Frequency response of the test amplifier. 

mately 6 decibels for injected pulses whose fre- 
quency differed from 15 kilocycles by 100 cycles. 
This discrimination of the amplifier against 
dopplered signals must be borne in mind when 
examining the test results, since these are 
stated in terms of the input level of the prim- 
audible pulse. No corrections for gain changes 
due to the tuning characteristic of the input 
circuit have been introduced in Figures 5, 6, 7, 
8, and 10 since the illustrated characteristic 
refers to sustained tones and may require 
further modification before it can be applied 
to pulses. 

Headphones 

The response characteristics of the head- 
phones and loudspeakers used are of impor- 
tance in evaluating the results. Unfortunately, 
no information is available concerning the 
loudspeakers, but the threshold curve for the 
headphones, reproduced in Figure 4, is worth 
examining. The points shown were obtained 
with three observers wearing each of three dif- 
ferent headsets in turn. These headsets were of 
the type used in the masking tests. The ob- 
served levels of just audible tones at each fre- 
quency are plotted in decibels relative to 
1.3 X 10~® volt. The rms amplitude of the faint- 
est 1-kilocycle tone that could be heard was 
4 X 10"® volt. 



EFFECT ON RECOGNITION 


239 


Use of sustained tones was necessary in de- 
termining the headphone threshold, since key 
clicks and transients made results obtained 
with pulses unstable and unreliable. Since the 
echo masking tests were not performed with 
sustained tones, the results shown in Figure 4 


plied by the first arrangement, but the altered 
procedure had practically no effect on the shape 
of the resultant threshold curve. It is not 
known, however, whether the response of these 
headphones is linear with input ; in other words, 
whether their relative response at various fre- 


UJ 

o 

< 



4 6 8 1 2 

100 


4681 2 4681 2 

1000 10*000 
FREQUENCY IN CYCLES 


Figure 4. Power of sustained tonal signals just audible at various frequencies, for the headphones used 
in the reverberation tests. 


may not be directly applicable. The level of 
room noise during the headphone study was 
somewhat higher than desirable and is believed 
to account for much of the scatter shown by the 
individual threshold determinations. However, 
the headphone tests extended over a period of 
several days, and the mean curve drawn 
through the points was considered reasonably 
free of the effects produced by extreme varia- 
tions in the level of room noise. 

The threshold shown in Figure 4 was ob- 
tained by matching the headphone impedance 
at 1 kilocycle to that of the signal generator. 
Since this procedure, which is the one followed 
in practice, neglects the possible effects of mis- 
match between headphone and generator im- 
pedances at other frequencies, a supplementary 
series of tests was made, in which nearly con- 
stant current was fed to the phones for a given 
generator setting at any frequency. This is in 
contrast to the nearly constant voltage sup- 


quencies is the same at comfortable listening 
levels as at the threshold of audibility. The 
mean voltage input to the headphones for the 
reverberation used in the masking tests, stabil- 
ized by means of automatic volume control as 
explained later, was 7X10-^ volt. Since this 
corresponds to a sensation level of 85 decibels 
[20 log (7X10-2/4X10-^)], which is somewhat 
higher than ordinarily considered comfortable, 
it may be inferred that the output of these 
headphones is not quite linear with input. 

The threshold curve shown in Figure 4 
agrees, in the region below 1 kilocycle, with 
that shown in Figures 76 through 79 in Chap- 
ter 4, although in both cases the threshold 
curve is much steeper than that computed on 
the basis of pressure in the ear canal. This 
departure is probably due to acoustic leakage 
at the low frequencies. In addition, however, 
the threshold shown in Figure 4 fails to drop 
in characteristic fashion between 1 and 4 kilo- 




RESTRICTED 


n 


240 


REVERBERATION MASKING OF SIGNALS WITH DOPPLER 


cycles, indicating that headphone response was 
somewhat unfavorable in that region ; and this 
indication should be borne in mind when as- 
sessing the relative degree of masking pro- 
duced by reverberation on signals with various 
amounts of up-and down-doppler (see Figure 
5, for example) . In the region of 10 kilocycles, 
the threshold curve in Figure 4 shows evidence 
of a resonance. Since this resonance frequency 
varied slightly among observers and for a 
given observer by changing the manner in 
which the headphones were held to the ears, 
there seems little doubt that the effect was 
due to a cavity resonance in the region of the 
outer ear. 

Volume Control 

Immediately following a transmission, the 
level of reverberation background is high. This 
level usually declines in a fluctuating manner 
over an interval of 2 to 3 seconds, depending 
on oceanographic and operating conditions, un- 
til the received background consists of ambient 
or self-noise. To prevent overloading of the 
receiver and detector by the early blast of re- 
verberation and yet have a gain setting high 
enough so that noise-limited echoes will be pre- 
sented at a favorable level, use of a level sta- 
bilizer is required.®"^^ At least three forms of 
stabilizer are in common use. The system used 
by the British is a modified type of automatic 
volume control and, in the tests under discus- 
sion, held the output level of the reverberation 
nearly constant to a range of about 2,500 yards. 

This is achieved by applying the average 
rectified output to the control grid of the first 
amplifier tube. In order to avoid application 
of bias while an echo is being received and 
the total level of the received sound is raised, 
application of bias is delayed for 100 millisec- 
onds, which exceeds the duration of most 
echoes. Removal of bias is made to occur 
within 10 milliseconds after the received level 
begins to fall, so that there will be no discrimi- 
nation against an echo which follows a rever- 
beration blob or which follows another echo. 
Owing to the 100-millisecond delay in applica- 
tion of bias, protection against the initial high 
level of reverberation is secured by an inde- 
pendent biasing arrangement and automatic 


volume control takes over about 100 to 200 
milliseconds after the transmission is com- 
pleted. 

The automatic gain control arrangement is 
designed to cease operating 6 seconds after a 
transmission and to resume only after another 
transmission. The need for this feature is im- 
posed by the fact that such automatic control 
would reduce the degree of output modulation 
in propeller sounds which the sonar operator 
may wish to hear. Similarly, it would reduce 
the contrast between sounds received on the 
hydrophone axis and slightly off its axis. 

Time-varied gain [TVG] and reverberation- 
controlled gain [RCG] are alternative forms of 
gain control. In the case of TVG, high bias is 
applied initially and allowed to taper off ex- 
ponentially. Thus, no drop of gain can be pro- 
duced by a high level blob just prior to an echo. 
With RCG systems, as with TVG, gain may 
increase but never decrease; however, the rate 
of gain restoration in the case of RCG is de- 
termined by the received level of the back- 
ground. Thus, when the reverberation level 
falls below that of noise, full gain is restored. 

Test Administration 

The tests were administered individually to 
between 2 and 6 observers most of whom were 
inexperienced. It was found, however, that 
after a few trials they gave consistent results 
which were in close agreement with the per- 
formance of experienced listeners. Headphone 
presentation was used in all cases except those 
described in Section 10.2.6, in which the ob- 
ject of the tests was to compare performance 
obtained with headphone and loudspeaker pres- 
entation. 

The level of primaudible pulses was deter- 
mined by means of an ascending series type of 
minimal increment test. Observers had no fore- 
knowledge of the range at which the signal 
was to be injected nor of the degree of doppler. 
These factors were held constant while the sig- 
nal level was brought up to primaudibility from 
a point well below threshold. When the ob- 
server had satisfied himself that he could de- 
tect no signal at a given level, gain in the sig- 
nal channel was increased by 3 decibels, and 
the test administrator noted the level of the 


EFFECT ON RECOGNITION 


241 


signal finally identified as primaudible in three 
out of five trials. To discourage guessing, each 
observer was required to indicate the range 
and, if possible, the direction of the doppler 
shift imparted to the signal. 

“Only one of the five observers who were 
asked to estimate the difference in frequency, 
if any, between a 70-millisecond [pulse] and 
the reverberations was able to do so with any 
accuracy. Three of the other four observers 
could distinguish a 20-cycle frequency change 
and state correctly whether it was high or low, 
but the remaining observer experienced great 
difficulty in deciding if the echo pitch was high 
or low, even when the frequency difference was 
as great as 60 cycles.”^ This variability in 
pitch discrimination (or rather, pitch identifi- 
cation) was apparently not strongly associated 
with the cue the observers used in these tests 
to identify the presence of an audible signal, 
since the difference in signal threshold levels 
for the best and poorest observers under given 
conditions rarely exceeded 6 decibels and was 
more often nearer 3 decibels. Furthermore, it 
is probable that some of the variability in rec- 
ognition differentials was produced by the fact 
that the reverberation samples presented to 
successive observers differed significantly 
among themselves. More specific information 
on ability to identify doppler was obtained in 
a different set of tests.^’^ 

After signal threshold levels had been deter- 
mined for each of the observers, the range, as 
well as the pulse frequency, was changed and 
thresholds were determined for the new con- 
dition. In this manner, the masking curves 
shown in Figures 5, 6, 7, 8, and 10 were de- 
termined. Each of the curves in these figures 
represents an independent test series. Slight 
differences in trends among these figures were 
produced by changes in the characteristics of 
the reverberation received from day to day 
during the tests. However, the data shown in 
any one figure were obtained on the same day, 
and over as short a time interval as possible, 
in order to minimize the effects due to changes 
in the received reverberation. It may be as- 
sumed therefore, that the effect of the chief 
variable indicated in each of the figures is rela- 
tively uninfluenced by instability of the mask- 
ing background. This assumption is strength- 


ened by the fairly high degree of consistency 
among the figures. 

The lower horizontal scales in Figures 5 
through 10 represent the difference between 
the frequencies of the pulse and the reverbera- 
tion. Thus, the positive values refer to up- 
doppler or closing-doppler, and the negative 
values to down-doppler or opening-doppler. 
The upper horizontal scale in Figure 5 trans- 
lates this frequency difference into units of 
relative target velocity, on the assumption that 
the emitted pulse has a frequency of 24 kilo- 
cycles [see equation (3)]. The vertical scale 
indicates the level of the primaudible pulse 
with the indicated degree of doppler. All the 
pulse threshold levels are expressed in decibels 
below an arbitrary reference, which was the 
same in all the tests. Thus, the figures do not 
give recognition differentials directly, but only 
changes in RD associated with doppler, pulse 
length, range, or heterodyne frequency. How- 
ever, estimates of recognition differentials 
which may be compared with those shown in 
Figure 12 of Chapter 9, are given in Section 
10.2.3. 

Recognition Differentials 

Figure 5 shows the effect of doppler in de- 
creasing the level of the primaudible signal. 


DOPPLER IN KNOTS FOR 24-KC TRANSMISSION FREQUENCY 
OPENING CLOSING 



Figure 5. Effect of doppler on the detectability of 
pulses masked by reverberation. 


The curve represents the mean performance 
of six observers and was determined in a rela- 
tively short time interval in order to offset 
effects produced by variation of the received 
reverberation with time. Range and bearing 


242 


REVERBERATION MASKING OF SIGNALS WITH DOPPLER 


were fixed in this series of tests and the trans- 
mission as well as the signal had a duration 
of 70 milliseconds. The heterodyne frequency 
was set so that the presented reverberation 
had a center frequency of 1 kilocycle. 

“The unsymmetrical character of the curve 
is most noticeable and the effect was readily 
demonstrated by introducing [a pulse] 100 
cycles higher than the reverberation frequency 
and reducing it in intensity until it was no 
longer audible. Then without altering the in- 
tensity, the [pulse] was plainly heard when it 
was reduced in frequency to 100 cycles lower 
than the reverberation frequency.’'^ In exam- 
ining this curve, allowance should be made for 
the fact that it was obtained with nearly rec- 
tangular pulses, which correspond essentially 
to beam-aspect echoes. In practice, submarine 
echoes will not show significant target doppler 
unless they are obtained at bow, stern, or quar- 
ter aspects. Under these circumstances, the 
echo envelope is generally inferior to what is 
observed at beam aspect, and the target 
strength also deteriorates for aspects other 
than beam. Similarly, the reverberations used 
in the masking tests were produced and re- 
ceived by a stationary transducer ; but, as 
pointed out previously, the width of the re- 
verberation spectrum is a function of lobe 
width, ship speed, and hydrophone orientation. 
Consequently, the reverberation spectra re- 
ceived in many cases of practical interest will 
be significantly wider than those which pro- 
duced the masking in the present set of tests. 

However, even when allowance is made for 
the effects of these factors, the final conclu- 
sions will depend ultimately on the ear’s per- 
formance in the case of dopplered and undop- 
plered echoes, as indicated in Figure 5. Such 
conclusions have considerable significance, 
since they form the basis for rules of proce- 
dure guiding prosubmarine and antisubmarine 
operations ; it is therefore desirable to examine 
the auditory results with some care. 

Perhaps the broadest basis for such an ex- 
amination is to compare the reverberation 
masking data with the results of tests for the 
masking of one sustained tone by another, as 
shown in Figure 2 of Chapter 2. In making 
such a comparison, it should be borne in mind 
that the sounds used in the present tests were 


not pure tones. The background had a spec- 
trum of finite width, and its time-amplitude 
pattern showed rapid variations of amplitude 
and phase. Similarly, the signal had a dura- 
tion of only 70 milliseconds. Thus, the essential 
widths of signal and reverberation spectra 
were about 29 cycles, and this is less than 50 
cycles, or the width of a critical band centered 
at 1 kilocycle. However, such sounds would be 
expected to give substantially constant stimula- 
tion of the basilar membrane over a frequency 
interval corresponding to a full critical band 
and to produce progressively less stimulation 
at more distant frequencies, as found in other 
cases of remote masking. It seems significant, 
therefore, that the width of the peak in Figure 
5 — as measured to points where the threshold 
level of the dopplered pulse is 6 decibels more 
favorable than for the undopplered — is about 
59 cycles, extending from about 23 cycles of 
down-doppler to about 36 cycles of up-doppler. 

In general this same peak width, approxi- 
mately that of a single critical band, would be 
expected for all cases in which the durations 
of emitted and received pulses exceed 70 milli- 
seconds, which is nearly twice the reciprocal 
of the critical band width at 1 kilocycle. Con- 
versely, the peak width should be greater than 
that of a critical band when the pulse is short- 
ened sufficiently so that its essential spectrum 
extends over an interval wider than a critical 
band, or, in other words, when the pulse length 
is less than about 70 milliseconds and the audio 
frequency is about 1 kilocycle. This dividing 
line occurs at about the pulse length where the 
RD for undopplered pulses becomes r^elatively 
independent of their durations (see Figure 11 
of Chapter 8 and Figure 14 of Chapter 9), and 
the inferred broadening of the peak, to a value 
of about 200 cycles for a 10-millisecond pulse, 
is indicated by the data in Figure 6. 

It will be clear, therefore, that the minimum 
condition for the resolution of echo and rever- 
beration spectra derived in Section 10.1 may 
not be applied unless these spectra exceed the 
width of the stimulated critical band, or, in 
general, exceed the resonance width of the af- 
fected receiver element. Thus, tests of the kind 
under discussion provide an additional method 
of determining critical band widths. Further- 
mor e, t he indications are that doppler shifts 




ESTRICTE 




EFFECT ON RECOGNITION 


243 


less than ±18 cycles (which amounts to one- 
half the width of the narrowest critical band, 
and also to about 1 knot of relative target 
motion for an echo-ranging frequency of 24 
kilocycles) cannot be detected by ear unless, 
perhaps, the audio frequency is reduced to 
about 200 cycles. 

For such low presentation frequencies, sub- 
jective harmonics are readily generated, and 
separation of the segments of basilar mem- 
brane stimulated by the harmonics is greater 
than for the directly presented sounds. This 
process should be even more effective for lower 
heterodyne frequencies ; but echoes with 
amounts of down-doppler exceeding the audio 
frequency would then emerge at the image 
frequency and might even appear to have up- 
doppler. Another obvious disadvantage of a 
low beat frequency is that abrupt shifts of 
phase associated with the irregular envelope of 
reverberation would become more noticeable to 
the ear as intervals of silence, so that the pre- 
sented background would acquire a rattling 
quality and might produce a deterioration in 
performance. With the periodmeter, this in- 
creased number of abrupt phase shifts with 
decreasing heterodyne frequency would show 
up as a broadening of the reverberation spec- 
trum. 

Conversely, the harmonics may be produced 
by means of a nonlinear circuit. For example, 
a square law circuit, such as is used in the 
doppler doubler, will multiply the frequencies 
of signal and background by a factor of two, 
thereby doubling any frequency difference be- 
tween them. However, such a mechanism, like 
the ear, will also produce the difference and 
the sum of the original frequencies. The sum 
frequency lies between the doubled values of 
the signal and background frequencies and^ 
especially when the signal is relatively weak, 
will tend, since it obviously cannot be filtered 
out, to obliterate the doubling effect, giving a 
sensed doppler no greater than available with- 
out the doubler. This difficulty can be avoided 
by recording the incoming sounds and then 
presenting them to the operator by running the 
sound track at increased speed but introduces 
a new difficulty in that the active duration of 
the echo is diminished. 


It should be explained at this point that the 
preceding estimate of ±18 cycles as the prac- 
tical lower limit for auditory determination of 
doppler shift should in general enable nearly 
certain identification of pitch differences, and 
that shifts of ±9 cycles should be detectable 
50 per cent of the time. Even this value, how- 
ever, is considerably greater than the smallest 
pitch differences which can be detected between 
pure tones (see Figure 10 in Chapter 2). This 
deterioration in performance is due to the brev- 
ity of the pulse and to variations in pitch and 
intensity of the background. Thus, trained ob- 
servers usually agree to within 3 cycles when 
they determine the salient frequency of a re- 
verberation sample by matching against an 
oscillator tone, provided the final setting of the 
oscillator is taken as the average of those ob- 
tained by raising and also dropping the tone 
frequency to the apparent midfrequency of the 
reverberation. This procedure eliminates the 
effects of finite pulse duration and spectral im- 
purity. Nonetheless, difficulty in obtaining a 
satisfactory match is experienced because the 
pitch of the reverberation fluctuates irregu- 
larly, and the observer must consciously try 
to find the midpoint of that zone. In addition, 
the mechanics of the basilar membrane some- 
times make it difficult to distinguish between 
changes of frequency and of intensity, although 
training minimizes the effects of such illusions 
and biases. Because of the changing intensity 
of reverberation background with time, and 
also because many reverberations show sys- 
tematic pitch drift, the most satisfactory ref- 
erence for estimating doppler is that portion 
of the background immediately preceding the 
echo. 

It may also be concluded that the pulse mask- 
ing curve in Figure 5 shows only a single peak 
because the sounds used, unlike those on which 
the sustained tone masking data shown in Fig- 
ure 2 of Chapter 2 are based, could establish no 
definite sensation of beats during the brief 
interval that the pulse was presented. How- 
ever, as pointed out in Section 9.2.1, there are 
substantial fluctuations in amplitude due to 
phase interference when pulse and reverbera- 
tion intensities are comparable. Since the in- 
tensity of a primaudible undopplered pulse, 
masked by CW reverberation, is about 7 deci- 


RESTRIGTE 


244 


REVERBERATION MASKING OF SIGNALS WITH DOPPLER 


bels higher than that of the reverberation for 
a pulse length of 70 milliseconds, Figure 5 im- 
plies that the envelope of the pulse-reverbera- 
tion mixture will not be substantially affected 
by interference except when signal doppler is 
±40 cycles and therefore the signal-to-rever- 
beration ratio nearly unity. The fact that the 
greatest degree of masking occurs for the pulse 
whose intrinsic frequency is equal to that of 
the reverberation indicates that a 70-millisec- 
ond signal stimulates the same portion of the 
basilar membrane as does a sustained tone of 
equal frequency. In other words, effects asso- 
ciated with the time required for a stimulus to 
affect different points on the basilar membrane 
do not appear significant for signals of this 
length and intrinsic frequency. 

While the threshold levels for pulses with 
small amounts of doppler are in good agree- 
ment with expectation, the fundamental sig- 
nificance of the relative thresholds for pulses 
with a doppler shift of 80 to 100 cycles is open 
to some question. There are several reasons for 
raising this question. One of these reasons is 
stated in the following paragraphs ; the others 
are given in Sections 10.2.5 and 10.2.6, in con- 
nection with the data discussed in those 
sections. 

The reason that the threshold levels for dop- 
plered pulses do not decline abruptly is that 
remote masking occurs ; that is, the finite 
spread of the disturbance on the basilar mem- 
brane produced by the reverberation renders 
dopplered pulses less audible than they would 
be in the absence of the reverberation back- 
ground. In a general way, and aside from the 
absence of the dip that appears when the tonal 
signal has nearly the same frequency as the 
tonal background. Figure 5 resembles the mask- 
ing curves obtained with sustained tones. This 
observation again implies that many of the re- 
sults based on tests with sustained sounds re- 
main at least approximately valid for the case 
of pulses (see also Section 8.1). 

In fact, if the curve shown in Figure 5 is cor- 
rected for two known sources of bias, the com- 
puted recognition differentials for 0.9-kilocycle 
and 1.1-kilocycle pulses are very nearly in exact 
agreement with the recognition differentials 
which may be derived from Figure 2 in Chap- 
ter 2 for sustained 0.9-kilocycle and 1.1-kilo- 


cycle tones masked by a 1-kilocycle tone which 
has a sensation level of 80 decibels. This sensa- 
tion level has been selected because the avail- 
able data imply that the reverberation used in 
the British tests was presented at approxi- 
mately this level. The two sources of bias re- 
ferred to are (1> the characteristic of the tun- 
ing curve shown in Figure 3 and (2) the some- 
what poorer response of the headphones for 
frequencies above 1 kilocycle, as indicated by 
the discussion of Figure 4. The computed recog- 
nition differentials for 0.9-kilocycle and 1.1- 
kilocycle tones masked by a 1-kilocycle tone are 
given in the middle column of Table 1, and the 


Table 1. Comparison between observed results 
and pure-tone data. 


Tone or pulse 

RD in db for 

RD in db for 70-milli- 

frequency 

tone masked 

second pulse masked 

in cycles 

by 1-kc tone 

by 1-kc reverberation 

900 

— 28 

— 23 

1,100 

— 19 

— 14 


observed recognition differentials for a 70-mil- 
lisecond pulse masked by 1-kilocycle reverbera- 
tion are given in the right-hand column. 

In deriving the ♦sustained-tone recognition 
differentials from Figure 2 of Chapter 2, the 
observations, originally expressed in terms of 
threshold shift, have been converted to decibels 
below the level of the masking tone. This has 
the effect of reducing somewhat the apparent 
degree of asymmetry between the masking of 
low- and high-frequency tones, because the 
level of the absolute audibility threshold falls in 
the interval between 0.9 and 1.1 kilocycles (see 
Figure 1 in Chapter 2). Also, it was necessary 
to interpolate among the observations made 
with an 0.8-kilocycle and 1.2-kilocycle masking 
.tone in order to estimate the effect for a 1-kilo- 
cycle masking tone. These derived recognition 
differentials for sustained tones are uncertain 
by perhaps as much as 3 decibels. The curves 
from which they were read have been smoothed, 
and can probably not be read more accurately 
than this. In examining the tabulated pulse 
recognition differentials, it should be noted that 
Figure 5 gives the threshold level of the dop- 
plered pulse in decibels below that of the prim- 
audible, undopplered pulse, and, as indicated in 


EFFECT ON RECOGNITION 


245 


Figure 12 of Chapter 9, the latter was itself 
some 8 decibels above that of the reverberation. 

In view of the uncertainties of measurement, 
and the assumptions required to arrive at the 
tabulated values, the agreement between the 
tone and pulse data is very satisfactory. Thus, 
the difference of 5 decibels between the two sets 
of recognition differentials is very nearly equal 
to the deterioration which would be expected to 
result from using a short pulse rather than a 
sustained tone. 

It is also of interest to use the pure tone 
data for purposes of estimating the probable 
effect of listening level, which is given in Table 
2. It may be concluded from Table 2 that recog- 
nition differentials for dopplered pulses will not 
be as favorable at low listening levels as at 
high. This conclusion is in agreement with the 
observation quoted in Section 9.2.2 for pulses 
without doppler, and is probably associated 
with the fact that smaller intensity changes 
can be detected at the higher listening levels. 
It will also be noted that the recognition differ- 
entials for 0.9-kilocycle and 1.1-kilocycle tones 
are more nearly equal at a listening level of 60 
phons than at a level of 80 phons. The loss in 


Table 2. Effect of loudness level on pure tone 
masking. 



RD in db for tone 

RD in db for tone 

Tone 

masked by 1-kc 

masked by 1-kc 

frequency 

tone presented at 

tone presented at 

in cycles 

60 phons 

80 phons 

900 

— 22 

— 28 

1,100 

— 17 

— 19 


audibility for the 0.9-kilocycle tone is somewhat 
greater than for the 1.1-kilocycle tone, presum- 
ably because the audibility threshold decreases 
with increasing frequency. Hence, the effective 
sensation level for the 0.9-kilocycle tone is 
lower than that for the 1.1-kilocycle tone. The 
loss of signal audibility of 2 to 6 decibels con- 
sequent on reduction of the listening level is of 
the expected order of magnitude (see Figure 
17 in Chapter 9) . 

With American echo-ranging gear, the 
sounds are heterodyned to 800 cycles. For a 
masking tone of this frequency, the pure tone 


recognition differentials, computed from Fig- 
ure 2 in Chapter 2, are listed in Table 3; so 
also are the results for masking tones with fre- 
quencies of 400 and 1,200 cycles. It will be 
seen from Table 3 that in general the recogni- 
tion differentials are more favorable at the 
higher listening level and that the difference in 
recognition differentials between the low- and 
high-frequency signals is smaller at the lower 
listening level. It should be remembered that 
these values are uncertain by several decibels. 
If the pure tone data shown in Table 3 apply 


Table 3. Recognition differentials for masking 
of tones by tones. 


Frequency in 
cycles of 
masking tone 

Frequency in 
cycles of 
masked tone 

RD in db 
at 

60 phons 

RD in db 
at 

80 phons 

400 

300 

— 33 

— 41 


500 

— 25 

— 25 

800 

700 

— 27 

— 34 


900 

— 18 

— 21 

1,200 

1,100 

— 16 

— 21 


1,300 

— 16 

— 17 


even approximately to dopplered echoes about 
100 milliseconds long and in the presence of 
reverberation, the indications are as follows. 
To begin with, dopplered echoes will be easier 
to detect at high listening levels and at low 
heterodyne frequencies. Secondly, the differ- 
ence in the detectability of echoes with opening 
and closing doppler will be smaller at the lower 
listening levels. Consequently, level stabilizers 
offer the advantages of making it possible to 
listen at optimal values of the gain setting. 
Furthermore, the doppler data shown in Figure 
5 are probably not reliable guides to perform- 
ance for gear which is not provided with A VC 
or RCG, not only because the echo may arrive 
when the level of reverberation has fallen to a 
low value, but also because dopplered echoes 
may be threshold rather than masking limited 
if the gain is set too low. Finally, it will be 
clear why differences in the spectra of rever- 
beration and rectangular undopplered pulses 
were able to depress the measured recognition 
differentials to negative values for the non- 
filtered pulses described in Section 9.1.2. 


/ ^R^RICTED 


246 


REVERBERATION MASKING OF SIGNALS WITH DOPPLER 


10.2.3 Recognition Differentials without 
Doppler 

During the tests described in the preceding 
section estimates were made of the primaudi- 
ble signal-to-reverberation ratio for pulses 
without doppler by the following two methods. 
In the first, a CRO with recurrent time base 
and linear deflection scale was used to portray 
the instantaneous voltage developed across the 
headphones, and a visual estimate was made of 
the relative amplitudes of the primaudible sig- 
nal and the reverberation at the range of pulse 
injection. In the second method, advantage was 
taken of the fact that the receiver was equipped 
with A VC, and the value of the control grid 
bias at the time of echo injection provided an 
indication of the relative levels of signal and 
reverberation at primaudibility. 

Since the level of the reverberation back- 
ground fluctuated over a wide range, in charac- 
teristic fashion, it was difficult to arrive at a 
very precise evaluation of the signal-to-rever- 
beration ratio corresponding to primaudibility, 
but both methods indicated that the pulse be- 
came consistently audible when its amplitude 
was nearly double that of the neighboring re- 
verberation peaks. This corresponds to an RD 
of 6 decibels for a 70-millisecond pulse without 
doppler (20 log 2). The report describing these 
tests^ also remarks that a primaudible signal- 
to-reverberation ratio of 2, as indicated above, 
“is a conservative estimate ; [pulses] of smaller 
amplitude may often be detected, possibly due 
to differences between the shapes of [pulses] 
and reverberation peaks.” It should be noted at 
this point that the estimates of signal-to-rever- 
beration ratio made by either of the methods 
used did not involve the difficulties usually at- 
tendant on visual detection of a signal masked 
by fluctuating background, since the test ad- 
ministrator knew the exact time of signal oc- 
currence. 

The RD of 6 decibels for an undopplered 
pulse 70 milliseconds in duration has been 
entered in Figure 12 of Chapter 9 as an open 
square; the same symbol is used to indicate 
the RD obtained in these tests for a 10-milli- 
second pulse without doppler, as described 
shortly. It will be seen that the squares are 


within 2 decibels of the line drawn through the 
filled-in circles which is well within the stand- 
ard deviation of recognition differentials deter- 
mined by the single-point method, and also well 
within the uncertainty of 3 decibels inherent in 
the present tests. Owing to the many sys- 
tematic differences between these British tests 
and those at UCDWR, closer agreement would 
not be expected. 

Comparison of signal threshold levels for 
durations of 10 and 70 milliseconds are shown 
in Figure 6. The data shown in this figure for 


2 

(D Qj 

< ul -10 


10-MI 

1 1 

LLISE 

rn 

:C0ND 

1 1 

PULS 

>E 





















70-M|LLISp0ND PULS 

E 


-100 -80 -60 -40 -20 0 20 40 60 80 

DIFFERENCE IN CYCLES BETWEEN PULSE 
AND REVERBERATION FREQUENCIES 


Figure 6. Effect of pulse length on the detecta- 
bility of dopplered pulses masked by reverberation. 


both pulse lengths were obtained within a rela- 
tively short time at a range of 500 yards and 
at fixed bearings. The bearing selected was not 
very critical in the case of the 70-millisecond 
pulse but, for the shorter transmission, the 
time-amplitude pattern and masking properties 
of the reverberation changed markedly with 
transducer bearing. In other words, echoes 
from small bottom features failed to overlap 
when the transmission length was made short 
enough, and the salient peaks in the received 
reverberation became difficult to distinguish 
from the signal, even when the latter had 
doppler. Consequently, the bearing used in the 
10-millisecond tests was selected to give the 
smoothest reverberations obtainable. 

If attention is confined to the threshold levels 
for 10-millisecond and 70-millisecond signals 
without doppler, it will be seen that the level 
of the primaudible signal was 4.5 decibels lower 
for the shorter pulse. Here, it should be men- 
tioned again that all signal levels are stated in 
terms of the same arbitrary reference stand- 
ard. Since the power output was independent 
of transmission length and since mean rever- 



EFFECT ON RECOGNITION 


247 


beration level for fixed conditions is inversely 
proportional to the length of the transmission, 
the level of the received reverberation was 
about 8.5 decibels (10 log 10/70) lower for the 
shorter transmission. However, detectability 
improved by 4.5 decibels when the 10-milli- 
second transmission was used; the assistance 
produced by the drop in reverberation level 
being offset in part by the reduced audibility of 
the shorter signal. Thus, the loss of audibility 
due to shortening the signal amounted to 4 
decibels. In other words, the RD for a 10-milli- 
second signal is 4 decibels higher than that for 
a 70-millisecond signal, as shown by the squares 
and the filled-in circles in Figure 12 of Chapter 
9. It should be noted that the listening levels 
were approximately equal for the tests at 10 
and 70 milliseconds due to the A VC action of 
the receiver. 

The slightly rising trend of signal threshold 
level with increasing pulse frequency which is 
shown by the 10-millisecond data in Figure 6 
disappears when correction is made for the 
response characteristics of the input circuit 
and the headphones. Instead, the corrected 
curve shows an improvement of 6 decibels re- 
sulting from 100 cycles of down-doppler, and 
an improvement of 4 decibels for 100 cycles of 
up-doppler; the corresponding recognition dif- 
ferentials are 4 and 6 decibels, respectively. In 
other words, the width of the signal threshold 
curve is nearly 200 cycles between points 6 
decibels down from the undopplered signal, and 
this is the theoretical width of the essential 
spectra of a 10-millisecond pulse and the rever- 
beration produced by it. For amounts of doppler 
exceeding the essential widths of signal and 
reverberation spectra, resolution of the two 
should be possible; in other words, recognition 
differentials for dopplered echoes, as well as 
ability to identify doppler, may be assumed to 
improve with increasing doppler. This improve- 
ment, however, will be limited by the fact that 
the essential spectra are wider for the shorter 
pulses and also by the fact that appreciable 
amounts of energy exist outside the limits of 
the essential spectra. Both of these factors 
restrict the applicability of Table 3, so that it 
is most reliable for the longer pulses. 


Effect of Range 

Figure 7 shows that substantially the same 
results are obtained at pulse injection ranges 
of 500, 1,000, and 1,500 yards. The tests on 



AND REVERBERATION FREQUENCIES 

Figure 7. Effect of injection range on the 

detectability of pulses. 

which this diagram is based were made over a 
relatively short time interval and with no more 
than 2 to 4 observers participating, in order to 
minimize effects due to change of reverberation 
level and character with time. The results are 
for a 70-millisecond pulse and a heterodyne fre- 
quency of 1 kilocycle. 

The level of the primaudible undopplered 
pulse is seen to diminish progressively with in- 
creasing range and in a manner corresponding 
to the progressive diminution of reverberation 
level with range. The average performance for 
all three ranges is in good agreement with the 
data shown in Figure 5 ; and the results for the 
three ranges agree among themselves to within 
1 to 2 decibels except for opening-doppler in 
excess of 50 cycles, where the scatter is about 
4 to 5 decibels. This scatter may perhaps result 
from the presence of noise masking. Signals 
with 100 cycles of down-doppler are likely to be 
masked at long range by noise components 
rather than by reverberation, since the prim- 
audible echo in the presence of reverberation 
only is more than 20 decibels below the rever- 
beration level and may therefore be weaker 
than the noise in a critical band. 

It may be noted that some confusion in echo 
identification is to be expected at ranges where 
the level of the reverberation is about equal to 
the noise level in a critical band centered at 
the reverberation frequency, and where the 


♦ 


'ESTRICTED 


248 


REVERBERATION MASKING OF SIGNALS WITH DOPPLER 


reverberation is at the point of being masked 
by the noise. For example, if a sudden surge 
of reverberation level occurs under these cir- 
cumstances, with the surge rising above the 
noise, the blob may easily be mistaken for an 
echo. No tests have been made under these con- 
ditions, but this point should perhaps be taken 
into account in training sonar operators. 

10.2.5 Effect of Heterodyne Frequency 

Since heterodyning subtracts a fixed num- 
ber of cycles from the frequencies of the echo 
and reverberation components, the frequency 
difference between the two is not affected by 
this process. Hence, the ratio of the doppler 
shift to the reverberation frequency increases 
as the heterodyne oscillator is set to give the 
reverberation a lower audio frequency. To 
determine whether the value of this ratio has 
an effect on the recognition differentials of 
dopplered echoes, a series of tests was per- 
formed in which the heterodyne setting was 
selected to present the reverberation at fre- 
quencies of 0.5, 1, and 1.5 kilocycles. The test 
sequence covered a short period of time. Pulse 
duration was 70 milliseconds, and range and 
bearing were fixed. The observations are plotted 
in Figure 8. 



DIFFERENCE IN CYCLES BETWEEN PULSE 
AND REVERBERATION FREQUENCIES 


Figure 8. Effect of heterodyne frequency on the 
detectability of pulses. 

The threshold level for an undopplered sig- 
nal is seen to vary by no more than ±1.5 
decibels for the three frequencies tried, an 
amount less than the experimental uncertainty. 
This near identity is what would be expected 
from the fact that detection of undopplered 
signals is based largely on intensity discrimi- 
nation, and this capacity does not depend 
strongly on frequency when the sensation level 
is fairly high (see Figure 15 in Chapter 2). 


The results for the 1-kilocycle setting are in 
good agreement with the data shown in Figure 
5. On the other hand, the effect of doppler, in 
the cases of the settings at 0.5 and 1.5 kilo- 
cycles, does not agree with expectation. Thus, 
the 0.5-kilocycle curve is more symmetrical and 
the 1.5-kilocycle curve less symmetrical than 
would be predicted from the sustained-tone 
recognition differentials shown in Table 3. This 
may mean that Table 3 is not a reliable in- 
dicator of pulse recognition differentials, since 
it does not take into account the spread of the 
reverberation spectrum outside the essential 
spectrum width A/ defined in Section 7.1. Al- 
though energy at these distant frequencies is 
negligible for most purposes, it may have some 
effect in the present case. The energy per cycle 
at any frequency / may be computed from 
equation (3) in Chapter 7. From this equation 
it is readily shown that the average energy 
per cycle in the spectral region between A//2 
and 3a//4 is about 23 decibels below the energy 
per cycle at the midfrequency /o. Thus for a 
70-millisecond pulse, with an essential spectrum 
width of 29 cycles, the average energy per 
cycle at a frequency 'of about 20 cycles (be- 
tween 15 and 22 cycles) away from the mid- 
frequency is 23 decibels down ; in the neighbor- 
hood of 100 cycles the spectrum level is an 
additional 13 decibels down (20 log 100/20), 
or 36 decibels down in all. While this seems too 
low to explain the observed results, the un- 
certainty in the measurements is such that 
this effect cannot be ruled out. 

In addition, the lack of improvement in the 
0.5-kilocycle tests may be due, in part, to 
acoustic leakage around the headphone caps. 
Thus, although the same voltage was applied 
to the headphones in all cases, the listening 
level was probably very much lower in the 0.5- 
kilocycle tests than in the others; as shown in 
Table 2, a lower listening level may be expected 
to impair echo recognition. It is possible that 
the primaudible 0.5-kilocycle pulses were not 
far above the absolute audibility threshold. 
Furthermore, interference from room noise 
was probably more of a factor for the tests at 
0.5 kilocycle, since most headphones do not 
efficiently insulate the ear from airborne sounds 
at the lower frequencies. Similarly, the en- 
hanced asymmetry in the 1.5-kilocycle curve 


^'^T RICTED 



EFFECT ON RECOGNITION 


249 


was probably caused, in part, by the unfavora- 
ble response of the headphones at frequencies 
above 1 kilocycle (see the discussion of Figure 
4). Finally, it is not known to what extent sys- 
tem response was affected by changes in 
heterodyne frequency. It may be that, if proper 
corrections could be introduced for this factor, 
the shapes of the curves in Figure 8 would be 
in better agreement with Table 3, and also that 
the peak widths for the various heterodyne 
frequencies would conform more closely to the 
critical band widths. 

It will be clear from the preceding discussion 
that recognition differentials for dopplered 
echoes masked by reverberation will be in- 
fluenced much more by system response than 
is likely to be true for noise masking. This dif- 
ference arises from the fact that, in the case of 
noise, the background components which pro- 
duce masking are usually at frequencies in the 
immediate neighborhood of the echo frequency ; 
hence, system response affects the levels of echo 
and masking components in much the same 
way. In the case of reverberation, on the other 
hand, dopplered echoes are subject to remote 
masking, that is, stimulation produced at points 
on the basilar membrane corresponding to fre- 
quencies higher and lower than that of the 
reverberation. This effect occurs within the 
ear; hence, a system which responds poorly at 
the echo frequency will be at a disadvantage 
because it discriminates against the echo but 
does not alter the amount of remote masking 
which takes place within the hearing mecha- 
nism. 

The upper diagram in Figure 9 represents 
schematically A the auditory threshold ; B 
a 1-kilocycle band of background noise admitted 
by a supersonic receiver and heterodyned to a 
center frequency of 1 kilocycle — this is shown 
as a shaded area and indicates the noise levels 
in 50-cycle bands; C the overall level of re- 
verberation received in the same system at the 
same time and at a range of about 500 yards, 
which is represented as a vertical bar; and D 
the levels of primaudible echoes, 70 milliseconds 
long and with various amounts of doppler, 
shown as a dashed curve. The system is as- 
sumed to have a flat response, as shown by E 
at the top of the figure. For the sake of sim- 
plicity, the reverberation side bands are not 


represented. It is clear from this diagram that 
increased echo doppler tends to mitigate the 
effects of remote masking and thereby to im- 
prove recognition differentials. With increasing 
range, the reverberation level declines, and 
weaker echoes can be recognized. The latter 



I 2 4681 2 4681 

100 1000 10,000 

FREQUENCY IN CYCLES 



100 1000 10,000 
FREQUENCY IN CYCLES 

Figure 9. Effect of system response on the 
detectability of dopplered echoes. 

condition is shown in the lower diagram, where 
everything is assumed to be the same as in the 
upper diagram, with the exception of the range 
or relative level of received reverberation. In 
this case, dopplered echoes are essentially noise 
masked; the effects of remote masking are 
negligible, and fainter echoes could be detected 
only if the level of received noise were reduced. 

The condition shown in the lower half of 
Figure 9 (level of reverberation presented to the 
ear not excessively high compared with back- 
ground noise) could in principle be maintained 
at all ranges by using a notch filter, or a series 




250 


REVERBERATION MASKING OF SIGNALS WITH DOPPLER 


of them, controlled by RCG. The notch filter is 
a bridged-T network, which can be made to dis- 
criminate very sharply against a narrow fre- 
quency band, without affecting response to fre- 
quencies removed only a few cycles from the 
limits of the notch. The depth of the notch, 
that is, the amount by which the level of the 
reverberation presented to the ear is attenu- 
ated with respect to the level of the received 
reverberation, can be controlled by an RCG 
circuit, in such a manner that the relations 
shown in the lower diagram are preserved at 
all times when appreciable amounts of rever- 
beration are received. It would probably be 
desirable to leave the reverberation distinctly 
audible, to provide a pitch reference for doppler 
judgment. 

Such a procedure would, of course, have no 
beneficial effect on the detectability of undop- 
plered echoes. In fact, it would probably raise 
the recognition differentials for such echoes by 
introducing distortion and by dropping the 
stimulation level at the reverberation fre- 
quency. Its advantage for dopplered echoes, 
which may become increasingly important for 
the high-speed submarine recently developed, 
is obvious from the lower diagram in Figure 9. 

In order to eliminate the effects of own- 
doppler, which would generally cause the fre- 
quency of the received reverberation to fall out- 
side the limits of the notch filter, it would be 
necessary to rely on ODN or some other 
method of stabilizing the pitch of the received 
reverberation. ODN is an electronic device which 
samples the frequency of the received rever- 
beration for a short interval immediately after 
each transmission. If the frequency of this 
short sample of reverberation is modified by 
own-doppler, the ODN unit alters the frequency 
of the heterodyne oscillator in such a manner 
that the output frequency of the reverberation 
at greater ranges is held to a constant value 
for a wide range of ship speeds and hydrophone 
orientations. 

10 . 2.6 Effects of Headphone and 
Loudspeaker Presentation 

These tests with headphones and loudspeaker 
presentation were undertaken in order to 
evaluate the significance of field reports re- 


ceived by the British which stated^ that “weak 
echoes are sometimes detected on the loud- 
speaker when they are inaudible in the [head- 
phones] Similarly, it had been reported “that, 
with echoes of fair strength, doppler is readily 
noticeable on the loudspeaker, while at the 
same time no doppler is reported from the 
operator using the [headphones] 

Sixteen observers were tested on their ability 
to hear 70-millisecond pulses with various 
amounts of doppler in the presence of rever- 
beration. The test procedure was the same as 
that described in the previous sections. Each 
observer was required to identify the level of 
the primaudible pulse, to state whether its fre- 
quency was the same as that of the reverbera- 
tion, and, if not, to specify whether its fre- 
quency was higher or lower than that of back- 
ground. In the present section, the discussion is 
concerned solely with the effect of loudspeaker 
and headphone presentation upon ability to 
detect dopplered and undopplered pulses. 

Range and bearing were fixed. Each listener 
made fourteen observations, corresponding to 
seven doppler conditions (0, ±20, ±40, and 
±60 cycles), repeated with loudspeaker and 
headphones. The amount of doppler was varied 
at random among the seven conditions stated, 
and the effect of variation in reverberation level 
with time was minimized by alternating fre- 
quently between loudspeaker and headphone 
presentation. Since the tests extended over a 
period of several days, the difference in per- 
formance obtained by the two presentation 



AND REVERBERATION FREQUENCIES 

Figure 10. Effect of loudspeaker and headphone 
presentation on the detectability of pulses. 

methods is much more significant than is the 
absolute level of performance. However, the 
latter, shown for both presentation methods in 
Figure 10, is in fair agreement with that 
shown in Figure 5. 


jvGlESTIfin'ED 



UCDWR DOPPLER TESTS 


251 


The loudspeaker and headphones used were 
standard service types. In order to simulate 
field conditions, the room in which the tests 
were made was not quieted in any way. The 
airborne sounds were produced largely by 
people walking in and through the test room, 
and the level, though not the character, of this 
room noise was judged to be similar to that 
obtaining on the bridge of an antisubmarine 
vessel. 

Figure 10 shows an advantage of about 1 to 3 
decibels for headphone presentation in the 
range between 60 cycles of down-doppler and 
10 cycles of up-doppler. For larger amounts of 
up-doppler, loudspeaker presentation improved 
performance by approximately 3 decibels. Thus, 
the test results confirm the indications obtained 
in the field. 

The differences are not large, however, and 
are probably due to a combination of factors. 
It has already been indicated in Section 10.2.1 
that the headphone response was somewhat un- 
favorable above 1 kilocycle. Furthermore, sen- 
sation levels and interference from room noise 
were probably significantly different for the 
two presentation methods. 

UCDWR DOPPLER TESTS 

Results of the UCDWR tests on reverbera- 
tion masking of dopplered pulses became avail- 
able too late to incorporate in and integrate 
with the preceding sections of this chapter. The 
observations and descriptive material upon 
which this summary is based constitute Part 
IV of a report^^ to be issued by the Sonar Data 
Division of UCDWR. Appreciation is expressed 
to the personnel of that organization for in- 
formally communicating these results in ad- 
vance of publication. The same document may 
be consulted for further details relating to 
some of the informally communicated material 
described in Chapters 8 and 9. 

Procedure 

The test backgrounds were obtained from 
playbacks of film-recorded sea reverberations 
produced in deep water by 36- and 114-milli- 
second CW transmissions from standard, 24- 
kilocycle echo-ranging gear mounted on a sur^ 


face vessel. The received reverberations were 
heterodyned to 800 cycles before recording, and 
all samples selected for use were free of signifi- 
cant pitch drift. The salient frequency of rever- 
beration samples was established by means of 
measurements on the film recordings and 
checked by aural matching to an oscillator tone. 
Results secured by these two methods agreed, 
on the average, to within 1 cycle. 

In all but one of the test series (see Section 
10.3.4) the signal was a nearly rectangular 
pulse, either 42 or 118 milliseconds long, ob- 
tained from an oscillator tone by means of a 
gating circuit. These pulse durations were con- 
sidered adequately matched to the reverbera- 
tions produced by the 36- and 114-millisecond 
transmissions, respectively. Pulses with a fixed 
amount of “doppler” (determined by controlling 
the frequency of the oscillator in the signal 
channel) were injected at a fixed range. How- 
ever, a number of different ranges were studied, 
all of them selected so that the signal occurred 
at a point beyond the initially overloaded sec- 
tion of the reverberation record, and several 
effects associated with this variable were estab- 
lished (see Section 10.3.3). Thus, some allow- 
ance can be made, when interpreting the 
present results, for the influence upon recogni- 
tion of foreknowledge of the injection range. 
While no similar basis exists for discounting 
effects produced by foreknowledge of the 
amount and direction of doppler, the general 
similarity between the UCDWR and the British 
data, which was obtained by randomizing the 
doppler shift, indicates that such effects are 
probably fairly small. 

The levels of successive pulses were varied 
at random among seven equally spaced values 
which extended over an interval of 14 decibels 
and which gave recognition probabilities vary- 
ing from nearly zero to nearly unity. Use of 
blank presentations and other precautionary 
measures effectively eliminated commissive 
errors. 

It may be mentioned in passing that differ- 
ence in pitch between the signal and the rever- 
beration context in which the signal occurs 
serves as an additional cue, so that the per- 
ceptual problem is somewhat more complex 
than in the no-doppler case. The momentary 
change of pitch during the life of the echo con- 


252 


REVERBERATION MASKING OF SIGNALS WITH DOPPLER 


stitutes a type of auditory motion. Thus, the 
“pitch wobble’" which characterizes most CW 
reverberation samples may occasionally result 
in multiple commissive errors when the ob- 
servers’ mental set is adjusted to doppler, while 
no such errors occur in no-doppler tests using 
the same reverberation background. Such com- 
missive errors, however, belong to a different 
category from the usual ones, since they do not 
necessarily represent errors of judgment. Re- 
verberation is composed of echoes from many 
discontinuities in the medium, some of them 
in motion. Probably as often as not there is no 
difference between the injected echo (or a 
target echo) and the constituent echoes of the 
background, other than the fact that one is 
wanted, and the remainder are not. The dis- 
tinction can be made in the field by noting 
whether an echolike portion of the received 
sounds recurs at a given range, by listening for 
propeller beats, and so on. 

Care was exercised in the design and opera- 
tion of the test apparatus, as well as in the 
handling and storage of film, in order to mini- 
mize the influence of extraneous noise. Such g 
effects were further reduced by transmitting 
the signal-background mixture through a band- ^ 
pass filter with cutoffs at 565 and 1,130 cycles, ^ 
which was sufficient to admit pulses with all ” 

degrees of doppler used in these tests. Har- ^ 

monic distortion in the signal channel probably ^ 
did not exceed 3 per cent for any condition J 
studied. 2 

The test sounds were presented to groups of 
five observers, the same group of five for any o 
one test series, by means of high-quality head- 5 
phones. All observers were young people, free § 
of hearing defects; in preliminary tests with g 
standardized material, they were found to rank 
with the highest 10 per cent of the population 
in pitch discrimination. For tests involving the 
effect of range (Figure 16), each doppler shift 
involved 300 judgments per observer, and for 
all other tests, between 1,000 and 1,500 judg- 
ments per observer. 

At the instant of signal injection, the mask- 
ing background generally had a sensation level 
of 55 to 65 decibels, and was at least 20 decibels 
above the threshold set by room noise. A small 
number of observations relating to the effect 
of listening level were made and are discussed 


below, but no absolute audibility thresholds were 
determined. Absence of information on audi- 
bility thresholds makes it impossible to express 
these data in terms of threshold shift and 
renders hazardous any attempts at precise com- 
parison between the present results and either 
the British doppler observations or studies of 
the masking of one sustained tone by another. 

Levels of signal and background were mea- 
sured independently, at the output of the 565- 
to 1,130-cycle band-pass filter, in accordance 
with the methods described in Section 9.2. 
Except for Figure 18, all recognition differ- 
entials reported here were obtained by the 
single point method and probably are directly 
comparable to the no-doppler recognition dif- 
ferentials shown in Figure 12 of Chapter 9. 
No recognition differentials were determined 
for undopplered pulses in the present tests. 



-14 -12 -10 -8 “6 - 4 , -2 0 2 

ECHO-TO-REVERBERATION RATIO IN 08 


Figure 11. Composite transition curve for a series 
of five tests with a 118-millisecond pulse given 40 
cycles of down-doppler. 

Instead, it was assumed that the RD for the 
no-doppler case could be read directly from the 
data for recorded 36- and 114-millisecond beam 
aspect echoes given in Chapter 9, and the posi- 
tions and shapes of the peaks of the curves 



UCDWR DOPPLER TESTS 


253 


shown in Figures 12, 13, and 16 were estimated 
in this way. Although the discussion in Sec- 
tion 9.1.2 indicates that this assumption may be 
questionable, the peaks of the curves in Figures 
12, 13, and 16 form reasonable continuations 
of the experimentally determined portions. It is 
not certain, however, that the recognition dif- 
ferentials for dopplered pulses were unaffected 
by click transients (see Section 10.3.4). 

Each of the recognition differentials shown 
in the various figures was determined from the 
50 per cent point of a composite transition 
curve representing the performance of the 
group in a specific test series. A typical tran- 
sition curve for a dopplered pulse masked by 
reverberation is shown in Figure 11. The shapes 
and slopes of transition curves for all degrees 
of doppler and both pulse lengths studied did 
not differ materially from those of the curve 
shown, and the latter is nearly an exact replica 
of the curve for a 114-millisecond beam aspect 
echo without doppler (see Figure 10 of Chap- 
ter 9). The standard deviations of recognition 
differentials for dopplered pulses were sub- 
stantially the same as found for the no-doppler 
case discussed in Section 9.2. 



REVERBERATION FREQUENCIES 

Figure 12. Effect of doppler on recognition differ- 
ential for 36-millisecond pulses at a range of 1,400 
yards. (Signal length equals 36 milliseconds; 
transmission length used in recording reverbera- 
tion, 42 milliseconds.) Dashed portion of curve was 
estimated from data obtained in no-doppler tests 
with recorded echoes. 

10.3.2 Effects of Signal Frequency and 
Duration 

The dependence of recognition upon doppler 
and pulse length is shown in Figures 12 and 13. 
The masking curves in these figures extend 
over a frequency interval of 400 cycles and 


apply to pulse lengths of 42 and 118 millisec- 
onds, respectively, injected at a range of 1,400 
yards. In both cases, the reverberation film 



DIFFERENCE IN CYCLES BETWEEN PULSE AND 
REVERBERATION FREQUENCIES 

Figure 13. Effect of doppler on recognition differ- 
ential for 118-millisecond pulses at a range of 1,400 
yards. Dashed portion of curve was estimated from 
data obtained in no-doppler tests with recorded 
echoes. 

loop consisted of five samples of background 
which had been carefully selected for uniform- 
ity of salient frequency. Because of small 
amounts of own-doppler and uncorrected 
changes in the setting of the heterodyne oscil- 
lator during recording, the average frequency 
characterizing a reverberation loop was not 
exactly 800 cycles ; the average frequency of the 
five samples on one reverberation loop was 770 
cycles, that of the other loop was 790 cycles. 
Similarly, the frequency difference between two 
reverberation samples on a loop never exceeded 
10 cycles. This frequency scatter, as well as the 
scatter among recognition differentials associ- 
ated with use of the single point method, is in- 
dicated by the circles in Figures 12 and 13. 
These points cluster in groups of five since they 
represent the results individually for each of 
the five reverberations on a loop. 

The results of the British and the UCDWR 
doppler studies are collated in Figures 14 and 
15. The following items should be borne in mind 
when examining these figures. First, the British 
data for 10- and 70-millisecond pulses apply to 
a reverberation frequency of 1,000 cycles pre- 
sented at a listening level of about 80 decibels, 
whereas the UCDWR curves (nominally, for 
pulse lengths of 36 and 114 milliseconds, since 


i 



254 


REVERBERATION MASKING OF SIGNALS WITH DOPPLER 


those durations were used to produce the rever- 
beration) represent a reverberation frequency 
of 800 cycles and a listening level of about 60 
decibels. Headphone response was probably sig- 
nificantly different in the two sets of tests 
(compare Figure 4 with Figure 76 in Chapter 
4) . There is, in addition, some evidence that 
noise components in the recorded backgrounds 
affected the results of the UCDWR tests (see 
Section 10.3.3), but that such effects were 



DIFFERENCE IN CYCLES BETWEEN PULSE AND 
REVERBERATION FREQUENCIES 


Figure 14. British and UCDWR masking curves 
showing effect of doppler on the detectability of 
pulses. 

absent or negligible in the British study (see 
Figure 7). Finally, it should be noted that, for 
the sake of removing the distorted appearance 
of the 10-millisecond results shown in Figure 
6, corrections have been applied to compensate 
for the effects of restricted receiver band width 
and the audibility threshold (see Figures 3 and 
4). The latter correction converts the 10-milli- 
second data into a masking audiogram, ex- 
pressed in terms of threshold shift. Although 
it would be desirable to treat the remaining 
three curves in similar fashion, not enough of 
the required information is available. 

The recognition differentials for the no- 
doppler condition shown in Figure 14 have 
been read from the curve in Figure 12 of Chap- 
ter 9. These values determine the relative posi- 
tions of the masking curves in Figure 14, which 
will be seen to form a reasonably consistent set. 


In agreement with the discussion in Sections 
10.2.2 and 10.2.3, the peak widths of the mask- 
ing curves tend to be equal to critical band 
width when the essential width of the spectrum 
is less than one critical band (70 and 114 milli- 
seconds) and equal to the essential widths of 
pulse and reverberation spectra when these 
exceed the width of the critical band at the fre- 
quency in question. It will also be observed 
that large doppler shifts reduce the masking 
(change the RD relative to the no-doppler condi- 
tion) by a greater amount for the longer pulse 
lengths. Both of these tendencies are to be ex- 
pected from the fact that the widths of signal 
and reverberation spectra (including side 
bands) increase with diminishing pulse length ; 
consequently, resolution of the two sounds be- 
comes increasingly difficult, that is, it requires 
greater separation of their nominal frequencies, 
and the results of pure tone masking tests be- 
come progressively less relevant. 

The shapes of the British and the UCDWR 
masking curves are significantly different, re- 
sulting in the intersection of the 70- and 114- 
millisecond curves. This difference in shape is 
probably due to the combined effects of differ- 
ences in (1) listening level (see Table 3), (2) 
spectral characteristics of the noise and rever- 
beration components in the masking back- 
grounds (see Section 10.3.3), and (3) pulse 
shapes used in producing the signal and rever- 
beration. 

To assist in evaluating the operational sig- 
nificance of the recognition differentials given 
in Figure 14, the curves have been replotted in 
Figure 15 to show the levels of the faintest 
audible echoes for a fixed set of conditions. 
Since the recognition differential equals 

10 log-^ = 10 log aS — 10 log B , 

B 

where B and S refer to the intensities of the 
masking background and the primaudible sig- 
nal, respectively; and, since B, in the case of 
reverberation background, is proportional to r, 
the transmission length, it follows that 

RD = 10 log 5—10 log kr, 

where /b is a constant characteristic of a par- 
ticular set of operating conditions. Thus, 

10 log 5 = + 10 log T + 10 log k. 


.■6 


STRICTED 


UCDWR DOPPLER TESTS 


255 


In other words, the ordinate in Figure 15, which 
represents the faintest detectable echo level, is 
derived by adding 10 log r to the ordinate for 
each of the curves in Figure 14 (see also Fig- 
ure 13 in Chapter 9). 

It will be clear from Figure 15 that fainter 
echoes can be detected in the presence of rever- 
beration for the longer pulse lengths, provided 
target doppler exceeds ±25 cycles (about 1.5 



DIFFERENCE IN CYCLES BETWEEN PULSE AND 
REVERBERATION FREQUENCIES 


Figure 15. Estimated improvement resulting from 
changes in doppler and pulse length when rever- 
beration is limiting. 


knots for an echo-ranging frequency of 24 
kilocycles) ; conversely, better results will be 
obtained in the case of stationary or slowly 
moving targets by reducing the pulse length. 
These conclusions refer specifically to auditory 
detection. They are further restricted at present 
by lack of detailed information relating to the 
effects of target aspect, of frequency and level 
stabilizers, of notch filters, and of heterodyne 
frequency. 


Effect of Range 

The effect of range on recognition of dop- 
plered pulses was studied by using a single re- 
verberation record produced by a 114-milli- 
second transmission. A 118-millisecond pulse 
with a fixed amount of doppler was injected at 
a fixed range, the only variable in a given test 


being the level of the injected pulse. Three test 
series were conducted, corresponding to ranges 
of 1,200, 1,600, and 2,000 yards. The recognition 
differentials obtained in these tests are shown 
in Figure 16. 



DIFFERENCE IN CYCLES BETWEEN PULSE AND 
REVERBERATION FREQUENCIES 

Figure 16. Effect of injection range on the 
detectability of dopplered pulses masked by 
reverberation. 

Previous studies of the learning effects en- 
countered in tests using a single reverberation 
sample as masking background indicated that 
such effects were probably substantially the 
same in all tests in this group ; hence, the three 
curves in Figure 16 are directly comparable 
with each other. Preliminary observations in- 
dicate that when the injection range is un- 
known the recognition differential may be as 
much as 2 to 3 decibels higher than when the 
range is known in advance. This effect is 
greater for the large doppler shifts than it is 
for the small, being no greater, on the average, 
than about 1 decibel for undopplered signals 
(see Section 9.2.2). The larger effect associ- 
ated with dopplered signals may arise from two 
circumstances. First, the perceptual situation 
is more complicated when the signal has a pitch 
different from that of the background (see Sec- 
tion 10.3.1). Secondly, the signal-to-reverbera- 
tion ratio is relatively small when the pulse has 
a large degree of doppler; in other words, the 
background is much louder than the primaudi- 
ble signal and more likely to divert attention 
from it. 

For this particular reverberation sample, 
there is a marked dependence of recognition 



256 


REVERBERATION MASKING OF SIGNALS WITH DOPPLER 


differential upon range. This dependence 
changes progressively with range, and is most 
noticeable for the larger amounts of doppler. 
It should be noted, however, that such range 
effects are observed in some cases but not in 
others (see Figure 7 and Section 10.3.4). 

It seems reasonable to infer that the differ- 
ence is due primarily to the presence of ap- 
preciable amounts of background noise at the 
ranges where these effects are observed. As in- 
dicated by the discussion of Figure 9, the char- 
acter of the sound reaching the operator after 
a transmission will change progressively with 
time as the reverberation decays. During this 
decay, the relative intensities of the various 
background components are gradually altered, 
and the nature of the masking is likewise 
modified. Obviously, therefore, attempts to 
classify background types by listening may be 
misleading when one of the components of the 
mixture has especially striking properties. 

The preceding discussion of the results 
shown in Figure 16 is supported by two other 
facts. First, for the larger doppler shifts the 
change of recognition differential with range 
is approximately equal to the change in level 
of the reverberation-noise mixture. In other 
words, as the background level falls, the remote 
masking of a dopplered pulse due to the rever- 
beration becomes smaller than the adjacent 
masking produced on that pulse by the rela- 
tively more prominent noise components. Sec- 
ondly, if this analysis is valid, the recognition 
differentials for pulses with large doppler 
injected at long range should be comparable 
with the recognition differentials for pulses 
masked by noise. The observed recognition dif- 
ferential for a pulse with 100 cycles of doppler 
injected at a range of 2,000 yards is, from Fig- 
ure 16, about —4 decibels. Assuming that the 
measured background may be regarded as 
equally distributed within a band 565 cycles 
wide (which was the pass band of the filter 
used in making the measurements), but that 
the masking was produced exclusively by the 
noise components in a 50-cycle critical band 
centered at the pulse frequency, gives a critical 
band RD which is about 10 decibels (that is, 10 
log 565/50) higher than that measured in the 
565-cycle presentation band, or 6 decibels. This 
is in good agreement with the critical band RD 


of 7 decibels indicated by Figure 8 in Chapter 
8 for a 114-millisecond pulse masked by noise. 

It is also worth comparing the results for 
the 118-millisecond pulse given in Figure 13 
with the results for the same pulse duration 
which are represented in Figure 16. The com- 
parison shows that the curve obtained in the 
former case, for a range of 1,400 yards, falls 
midway between the curves for 1,200 and 1,600 
yards in the second figure. In other words, 
pulses with large degrees of doppler in Figure 
13 were apparently subject to some amount of 
noise masking, which may in part account for 
the intersection between this curve and the 70- 
millisecond curve in Figure 14. No plausible 
reason can at present be assigned for the con- 
stancy of the RD for up-doppler values between 
100 and 200 cycles shown in Figure 13. 

10.3.4 Detection of Recorded Echo 

To add realism, a limited amount of data was 
obtained with a recorded 114-millisecond beam- 
aspect echo of constant intrinsic frequency. In 
this preliminary program the procedure which 
was followed (the only feasible plan in the time 
available) consisted of introducing doppler by 
using the single echo mentioned, as signal, and 
a reverberation background loop containing 
eight samples of different salient frequency and 
also produced by a 114-millisecond transmis- 
sion. Of these eight, two had frequencies 
matched to that of the echo, two were of lower 
frequency, that is, the signal had different 
amounts of down-doppler with respect to them, 
and four gave various degrees of up-doppler. 
The playback from such a test loop resembles 
the sequence of reverberations heard aboard a 
moving antisubmarine vessel not equipped with 
ODN, when the axis of the hydrophone is 
trained to successive orientations during a 
search. It may be useful to simulate this prac- 
tical situation more closely in further tests by 
varying the frequency of the signal as well as 
that of background. The total range of con- 
ditions tested in the manner described ex- 
tended from 80 cycles of opening to 80 cycles 
of closing-doppler. 

In order to reduce the effect of learning, 
since each sample on the background loop gave 
only one doppler condition, the echo was alter- 


HKSTKICTED 


UCDWR DOPPLER TESTS 


257 


nately injected at one of three predetermined 
ranges in each test. Unlike those already dis- 
cussed, this group of tests showed no systematic 
dependence on range. Hence, the points shown 
in the results, graphed in Figures 17 and 18, 
represent averages for the three ranges used. 


Since an RD determined by the single-point 
method for a single reverberation, even at three 
ranges, may have a low statistical reliability, 
the background levels were redetermined plani- 
metrically by measuring over and prior to the 
echo. The recomputed recognition differentials. 



-150 -100 -50 0 50 100 150 


DIFFERENCE IN CYCLES BETWEEN PULSE 
AND REVERBERATION FREQUENCIES 



-150 -100 -50 0 50 100 150 


DIFFERENCE IN CYCLES BETWEEN PULSE 
AND REVERBERATION FREQUENCIES 


Figure 17. Effect of doppler upon detection of a 
recorded beam-aspect echo 114 milliseconds long. 
Doppler was produced by using recorded reverbera- 
tion backgrounds whose pitch differed from that of 
the echo. Reverberation levels were measured by 
the single-point method ; in the case of the dashed 
curve, the level of background was measured at the 
output of a 50-cycle band-pass filter centered at 
the echo frequency. 

The horizontal scale of these figures specifies 
the amount and direction of the frequency dif- 
ference between echo and background, referred 
to the frequency of the background. The ver- 
tical scale gives the recognition differential ob- 
tained for each doppler condition. 

The tests yielded three principal — and un- 
expected, or perhaps, anomalous — results. 
First, the curve defined by the circles in Figure 
17, which gives the recognition differentials as 
determined by the single-point method, is asym- 
metrical in the opposite sense to those previ- 
ously encountered. Secondly, doppler improves 
recognition much less than in the cases already 
described. Finally, in a few tests in which the 
listening level was raised by about 15 decibels 
above the 55- to 65-decibel level employed in 
the bulk of this series, the higher level impaired 
recognition by about 1 to 2 decibels. 


Figure 18. Effect of doppler upon detection of a 
recorded beam-aspect echo 114 milliseconds long. 
Doppler was produced by using reverberation back- 
grounds whose pitch differed from that of the echo. 
Reverberation levels were obtained by planimetry ; 
in the case of the dashed curve, the level of back- 
ground was measured at the output of a 50-cycle 
band-pass filter centered at the echo frequency. 

shown by the circles in Figure 18, do not differ 
significantly from the single-point values, 
which appears to eliminate this factor as an 
explanation of the disagreement between the 
present and previous results. 

Since each of the doppler shifts was associ- 
ated with a different reverberation sample, 
while, in the other tests, each doppler shift was 
paired with, and the results averaged over, all 
of the samples, it was decided to investigate 
the possibility that the discrepancies arose from 
differences in the relative amounts of energy 
contained in the various samples at the echo 
frequency, that is, from differences in the 
shapes of their spectra. For this purpose, the 
reverberation level measurements were re- 
peated using a 50-cycle, instead of a 565-cycle 
filter centered at the echo frequency, on the 
assumption that the masking was produced 
essentially by the background energy in a 50- 



258 


REVERBERATION MASKING OF SIGNALS WITH DOPPLER 


cycle critical band. The single-point and plani- 
metric recognition differentials recomputed on 
this basis are given by the triangles in Figures 
17 and 18 respectively. 

It will be seen that planimetry reduces the 
scatter among the triangles, yielding a smoother 
curve. In addition, the circle and triangle for 
the no-doppler case coincide in Figure 18 but 
do not in Figure 17. The latter result is pre- 
sumably due to the fact that the narrow filter 
rounds the reverberation envelopes, removing 
the more prominent amplitude peaks. In con- 
sequence, the single-point no-doppler recogni- 
tion differentials based on measurements with 
a constrictive filter centered at the reverbera- 
tion frequency would be expected to exceed 
those based on wide-band measurements. On 
the other hand, averaging the reverberation 
level over and prior to the echo should reduce 
the influence of envelope rounding and bring 
the wide- and narrow-band measurements into 
better agreement. 

The dashed lines in Figures 17 and 18 curve 
upward from the no-doppler value. In other 
words, the ratio of primaudible signal to re- 
verberation, both measured in a 50-cycle band 
centered at the signal frequency, is greater 
when signal and background differ in fre- 
quency. If this ratio were the same for all the 
frequency conditions tested it would follow 
that the process under consideration involves 
adjacent masking; that is, that masking arises 
primarily from the background components 
whose frequencies coincide with those in the 
essential spectrum of the signal. Thus, it may 
be concluded from Figures 17 and 18 that the 
masking is adjacent in the no-doppler case, 
but remote when significant doppler exists. 
Remote masking, however, is produced by the 
energy in the essential spectrum of the rever- 
beration. Since this makes the dominant con- 
tribution to the level measured in the 565-cycle 
band, such measurements should provide an 
adequate basis for evaluating recognition dif- 
ferentials. Consequently, differences in the 
shapes of the reverberation spectra do not ac- 
count for the departure of the present results 
from expectation. It may be inferred, inci- 
dentally, that the frequency discriminating 
mechanism of the ear does not have such sharp 


cutoffs as those of the 50-cycle filter used in 
these measurements. 

If it could be demonstrated that the data 
given in Figures 17 and 18 are more represen- 
tative of field conditions than those illustrated 
in Figure 14, that conclusion would have an 
immediate and obvious bearing upon choice of 
tactics. Thus, a submarine taking evasive action 
would be less detectable at a given speed and 
range when presenting its stern to a searching 
antisubmarine vessel than when presenting its 
bow. However, the preceding analysis does not 
by any means eliminate all the difficulties 
which must be removed before Figures 17 and 
18 may be considered reliable. 

It may be, for example, that the effect shown 
in these figures is real, that it will be met when- 
ever the pitch of successive reverberations 
changes randomly, but that it will not be 
encountered in the absence of own-doppler 
shifts. To some extent, such effects are to be 
expected because the amount of remote mask- 
ing produced by a background tone upon a sig- 
nal which differs from it by a fixed number of 
cycles depends upon the frequency of the back- 
ground (see Table 3), and this fact is ignored 
when, as in Figures 17 and 18, the frequency 
of background is treated as fixed. 

Of course, the justification for following the 
procedure used in constructing Figures 17 and 
18 lies in the fact that the range of variation 
of background frequencies is small. Hence, the 
masking curves should be much the same for 
all of them. Similarly, the effects of variation 
in auditory threshold, sensation level, and am- 
plitude and frequency response of the test ap- 
paratus should be relatively small over the 
range studied. In addition, it seems improbable 
that any confusion produced by irregular 
changes in pitch among successive background 
samples could have any large effect on their 
ability to mask a signal, although it might be 
more difficult to make reliable judgments of 
the degree and direction of doppler under such 
circumstances. 

Finally, it seems possible that the reason for 
the disagreement between Figures 14 and 18 
is to be found in the properties (e.g., spectrum, 
envelope) of the recorded echo used as signal 
in the latter tests. In that case useful informa- 
tion may perhaps be obtained by repeating the 


RESTRICTED 


D 


UCDWR DOPPLER TESTS 


259 


variable reverberation pitch tests, and sub- 
stituting a 114-millisecond pulse for the re- 
corded echo. If thi^ procedure proved helpful, 
further useful information might be obtained 
by modifying the pulse in various ways to 


simulate the commoner types of echoes. Clearly, 
it would be simpler to repeat the set of variable 
background items than it would be to select and 
control a new group of recorded echoes of 
variable pitch. 



Chapter 11 

SUMMARY OF ECHO STUDIES 


1 BASIC FACTORS 

4 s IN LISTENING to target sounds, a target 
JLl- echo can be heard only if it is sufficiently 
intense compared to the background of un- 
wanted sounds. The echo level which can be 
heard half the time is called the echo recogni- 
tion level. The difference in decibels between 
the echo recognition level and the background 
level, measured in some specified band, is called 
the recognition differential. In echo-ranging 
applications, a 1-kilocycle band is frequently 
used for specifying the background level. 


Background Characteristics 

The background will generally include both 
noise — which may be airborne noise, electrical 
noise, or water noise — and reverberation 
produced by scattering of the pulse at the sea 
surface, sea bottom, and from scatterers in the 
ocean itself. The properties of noise are sum- 
marized in Chapter 6; for echo-ranging pur- 
poses, the spectrum level of noise in the water 
may usually be considered independent of fre- 
quency in the listening band. The properties of 
reverberation which are significant in echo rec- 
ognition are summarized below. 


^ Echo Characteristics 

The following characteristics of echoes are 
significant in acoustic detection. 


Spectrum 

The sound power of the echo in each 1-cycle 
band is nearly proportional to the power of the 
outgoing pulse in the same band, except for a 
uniform doppler shift caused by the relative 
velocity of the echo-ranging projector and the 
target. For an ideal square-topped pulse r sec- 
onds long, about 90 per cent of the energy is 
contained in a band 2/t cycles wide, centered at 
the nominal frequency of the pulse. Thus the 
“essential spectrum” of an echo from such a 
pulse is confined in a band 2/t cycles wide. 


Amplitude 

Echoes from a small target or from a vessel 
at beam aspect will usually be very similar in 
shape to the outgoing pulse. However, echoes 
from targets which have appreciable extent in 
the direction of the sound beam may be con- 
siderably prolonged compared with the out- 
going pulse, and their envelopes may be very 
irregular. 


Spectrum 

For a stationary echo-ranging projector, the 
sound power per cycle in the received rever- 
beration should be proportional to the power 
per cycle in the outgoing pulse. The phases of 
the different spectral components in the pulse 
and in the reverberation will be quite different, 
however. When the projector is moving, the 
change of own-doppler with relative bearing 
will broaden the reverberation spectrum unless 
the projector is pointed dead ahead or astern. 
Thus the essential width of the reverberation 
spectrum is 2/t cycles, plus an amount which 
depends both on the ship speed, the width of 
the main lobe of the transducer, and the rela- 
tive bearing of the transducer. When a pulse 
of varying frequency is sent out, the rever- 
beration spectrum covers the entire band of 
frequencies swept by the pulse, and doppler 
cannot be distinguished. 

Amplitude 

The reverberation intensity (or amplitude 
squared) is directly proportional to the pulse 
length at any fixed range which is much greater 
than the pulse length. The reverberation in- 
tensity decreases with increasing range, and, 
with present gear mounted aboard a ship 


260 



RECOGNITION LEVELS 


261 


moving in deep water, usually falls below the 
noise level at a range between 1,000 and 2,000 
yards. 

The time-amplitude pattern of reverberation 
is very irregular. The average duration of a 
reverberation blob produced by a single-fre- 
quency pulse tends to be about the same as the 
durations of pulse and echo, thus adding to the 
difficulties of echo recognition. When a pulse of 
varying frequency is sent out, the duration of 
a reverberation blob becomes much shorter, 
while the length of the returning echo remains 
unchanged. 

11.1.3 Echo-Background Mixture 

The echo-background mixture should be pre- 
sented at a loudness level in the neighborhood 
of 70 phons for best aural results. To achieve 
a constant loudness level, some form of variable 
amplification is required, since the reverbera- 
tion intensity decreases with range. The listen- 
ing band should be wide enough to pass echoes 
with any likely doppler shift, and, if very 
short pulses are used, should be at least 2/r 
cycles wide in order to pass the essential 
spectrum of the echo. Moderate limiting in the 
electrical circuit does not seem to have any 
significant effect on echo recognizability. 

112 RECOGNITION LEVELS 

Noise Background 

Recognition differentials for 800-cycle pulses 
are given in Table 1. The recognition differ- 


Table 1. Recognition differentials for 800-cycle 
pulses. 


Pulse length 
in seconds 

RD in db for a 

1-kc band 

1 or more 

— 14 

0.5 

— 12 

0.2 

— 9 

0.1 

— 7 

0.05 

— 4 

0.02 

0 

0.01 

4 

0.001 

16 


entials are computed in terms of a 1-kilocycle 
band width. Narrowing the band width does 
not affect the pulse recognition level, provided 


the noise band width exceeds both the critical 
band width of the ear and the essential width 
of the pulse spectrum. Such a decrease in band 
width, however, increases the computed rec- 
ognition differential, since the measured noise 
level decreases. The values in this table may be 
used for echoes from small objects and from 
ships at beam aspect and are probably valid 
for rounded as well as for square-topped echoes. 
For extended, irregular echoes, considerable 
deviations from these values may be expected. 

For the shorter pulses, these values are also 
applicable to other heterodyne frequencies. At 
the longer pulse lengths, the recognition differ- 
entials are modified by the change of the ear’s 
critical band width with frequency. Recognition 
differentials for long pulses of different fre- 
quencies, lasting 1 second or more, are given 
in Table 2. With mechanical or visual presenta- 


Table 2. Recognition differentials for long pulses 
of different frequencies. 


Heterodyne frequency 
in cycles 

RD in db for a 

1-kc band 

200 

— 14.6 

400 

— 14.6 

600 

— 14.6 

800 

— 14.2 

1,000 

— 13.8 

1,500 

— 12.7 

2,000 

— 11.5 

4,000 

— 8.0 


tion, the recognition differentials shown in 
Tables 1 and 2 can be equaled only by use of a 
number of filters, similar to the critical bands 
of the ear. When recognition differentials for 
aural and visual presentation are about the 
same, a gain of about 2 decibels is obtained by 
use of both presentations simultaneously. 

Repeated Pulses 

Use of two pulses spaced about half a second 
apart gives an RD about 3 decibels lower (more 
favorable) than for a single pulse. If a string 
of pulses is sent out, successive pulses tend to 
merge into a steady tone if the pulse repetition 
frequency exceeds about 20 times per second. 
If the echo intensity is averaged over the dura- 


7 


RESTRICTED 


262 


REVERBERATION MASKING OF SIGNALS WITH DOPPLER 


tion of the pulse string and the noise level is 
measured in a 1-kilocycle band, the recognition 
differentials given in Table 2 for long pulses, 
or sustained tones, are applicable. For a repeti- 
tion frequency less than 20 times per second, 
the recognition differentials computed with the 
peak echo intensity will be within 6 decibels 
of the values given in Table 1. 

Frequency-Modulated Pulses 

For a pulse at least 100 milliseconds long, 
the RD for a pulse of steadily increasing or de- 
creasing frequency (chirp signal) is within 2 
decibels of the RD for a constant-frequency 
pulse of the same duration. This conclusion 
presumably holds only if the frequency sweep 
does not extend much above 2 kilocycles, since 
at these higher frequencies increasing critical 
band width impairs performance. 

Reverberation Background without 
Doppler 

Recognition differentials for 800-cycle pulses, 
presented against reverberation of the same 
frequency, are given in Table 3. These rec- 


Table 3. Recognition differentials for undop- 
plered pulses masked by reverberation. 


Pulse length 
in seconds 

RD in db relative to 
reverberation 

Echo recognition 
level in db 

0.5 

2 

0 

0.2 

5 

— 1 

0.1 

7 

— 2 

0.05 

9 

— 3 

0.02 

11 

— 5 

0.01 

12 

— 6 


ognition differentials are independent of the 
presentation band width, provided this exceeds 
the essential width of echo and reverberation 
spectrum. 

To estimate actual performance, the propor- 
tionality of reverberation intensity and pulse 
length must be taken into account. This im- 
provement in performance is shown in the 
third column of Table 3, which gives echo rec- 
ognition levels relative to the recognition level 
for a 0.5-second pulse. 


The values of Table 3 are applicable to 
echoes from small targets and from vessels at 
beam aspect. Echoes which have highly ir- 
regular envelopes or which are prolonged rela- 
tive to the outgoing pulse may show recognition 
differentials several decibels more or less than 
the values given in the table. Subject to this 
restriction, these values may be used for all 
heterodyne frequencies between 0.5 and 1.5 
kilocycles; for frequencies outside this range, 
the validity of Table 3 is questionable. It may 
be noted that the simple types of visual and 
mechanical recognition would be expected to 
yield an RD less dependent on pulse length in 
the presence of reverberation, amounting to be- 
tween 5 and 10 decibels. 

For FM pulses presented against a rever- 
beration background, the RD for aural recogni- 
tion is probably in the neighborhood of 5 
decibels less (more favorable) than for pulses 
of constant frequency and the same length, as 
long as the pulse length is not too short; at 
about 20 milliseconds, for example, the relative 
advantage of the frequency sweep probably 
disappears. Even lower echo recognition levels 
are probably obtainable by use of complex 
visual or mechanical systems with short FM 
pulses and suitable averaging and filtering cir- 
cuits. 

^ Reverberation Background with 
Doppler 

With aural detection the recognition of 
echoes against reverberation is much facilitated 
by the doppler shift of echo relative to rever- 
beration. For echoes and reverberation pro- 

Table 4. Recognition differentials for dopplered 

pulses masked by reverberation. 


RD in db above reverberation 
Doppler shift for a 70-millisecond pulse 

in cycles Up-doppler Down-doppler 


0 

7 

7 

20 

3 

1 

40 

— 1 

— 6 

60 

— 6 

— 13 

80 

— 10 

— 18 

100 

— 13 

— 24 


duced with a 70-millisecond pulse, the recogni- 
tion differentials for different doppler shifts 
are given in Table 4. 


RECOGNITION LEVELS 


263 


Table 4 applies strictly only to 70-millisecond 
pulses injected in reverberation. The relative 
change of RD with doppler shift shown in this 
table is believed to be more generally applica- 
ble, however, since this depends only on the 
filter properties of the ear. Thus, for all 
echoes, even distorted ones of all pulse lengths 
greater than 70 milliseconds, the change of RD 
with changing doppler shift may be taken from 
Table 4. For shorter pulse lengths, however, 
the essential widths of pulse and reverberation 
spectra increase so much that the advantage of 
doppler is lost. At 10 milliseconds, for example, 
the RD is nearly independent of doppler for 
shifts up to 100 cycles. 

The loudness level infiuences considerably the 
recognition differentials obtainable with dop- 
plered echoes. Table 3 gives values for a uni- 


form loudness level of 70 phons for the pre- 
sented reverberation. For lower loudness levels, 
it is probable that the difference between up- 
doppler and down-doppler diminishes, and the 
recognition differentials for a fixed doppler 
shift becomes greater (less favorable). 

Improvement of performance for dopplered 
echoes produced with a pulse length greater 
than about 50 milliseconds could presumably 
be obtained with a notch filter, with a cutoff 
sharper than that of the ear’s critical bands. 
A notch filter would be required with visual or 
mechanical presentation to give performance 
comparable with that shown in Table 4."^ 

“ N.B. Useful concepts, whose meaning is devel- 
oped at diverse points in the preceding pages, have 
been indexed under unified headings to facilitate synopsis 
and review. 


. 3 : 



1 


,*■ V 

-V 


A I 


r 





'■I 










BIBLIOGRAPHY 


Numbers such as Div. 6-510. 12-M2 indicate that the document listed has been microfilmed and that its 
title appears in the microfilmed index printed in a separate volume. For access to the index volume and to 
the microfilm, consult the Army or Navy agency listed on the reverse of the half-title page. 


Chapter 1 

1. Speech and Hearing, Harvey Fletcher. D. Van 
Nostrand Company, New York. 1929. 

2. Hearing in Man and Animals, R. T. Beatty. Bell, 
London. 1932. 

3. Theories of Sensation, A. F. Rawdon-Smith. Cam- 
bridge University Press. 1938. 

4. Hearing, S. S. Stevens and H. Davis. John Wiley 
and Sons, New York. 1938. 

5. An Introduction to Biophysics, O. Stuhlmann, Jr. 
John Wiley and Sons, New York. 1943. 

6. Vibration and Sound, P. M. Morse. McGraw-Hill 
Book Company, New York. 1936. 

7. A Textbook of Sound, A. B. Wood. Macmillan Com- 
pany, New York. 1941. 

8. Direct Observation of the Acoustic Oscillations of 
the Human Ear, H. G. Kobrak. Journal of the 
Acoustical Society of America, Vol. 15, 1943, p. 54. 

9. “Theory of the Mechanical Phenomena Occurring 
in the Internal Ear,” J. A. Reboul. Journal de 
Physique et le Radium, Vol. 9, 1938, p. 185. 

10. “Theory of Hearing, Form of the Vibration Pattern 
of the Basilar Membrane,” G. v. Bekesy. Physi- 
kalische Zeitschi'ift, Vol. 29, 1928, p. 793. 

11. “Resonance in the External Auditory Meatus,” N. 
Fleming. Nature, Vol. 143, 1939, p. 642. 

12. “The Pressure Distribution in the Auditory Canal 
in a Progressive Sound Field,” F. M. Wiener. 
Journal of the Acoustical Society of America, Vol. 
18, 1946, p. 248. 

13. “Resonance in the External Auditory Meatus,” T. 

S. Littler. Nature, Vol. 143, 1939, p. 118. 

14. “Altered Acoustical Resonance of the External Au- 
ditory Canal as a Factor in the Transient Improve- 
ment of Hearing by Fenestration Operations in 
Chronic Progressive Deafness,” E. M. Josephson. 
Journal of the Acoustical Society of America, Vol. 
13, 1941, p. 83. 

15. “Aero-Otitis Media in Submarine Personnel,” H. L. 
Haines. Journal of the Acoustical Society of Amer- 
ica, Vol. 17, 1945, p. 136. 


Chapter 2 

1. “The Effect of High Altitude on Speech and Hear- 
ing,” H. W. Rudmose, K, C. Clark, F. D. Carlson, 
J. C. Eisenstein, and R. A. Walker, Journal of the 
Acoustical Society of America, Vol. 18, 1946, p. 250. 

2. “Experiments on the Pellet-Type of Artificial 
Drum,” K. Lowy, Journal of the Acoustical Society 
of America, Vol. 13, 1942, p. 383. 

3. “Immobilization of the Round Window Membrane, 
A Further Experimental Study,” W. Hughson and 
S. J. Crowe. Annals of Otology, Rhinology and 
Laryngology, Vol. 41, 1932, p. 332. 

4. “Reception of Sound by the Outer Ear,” J. Troger, 
Physikalische Zeitschrift, Vol. 31, 1930, p. 26. 

5. “On Minimum Audible Sound Fields,” L. J. Sivian 
and S. D. White, The Journal of the Acoustical So- 
ciety of America, Vol. 4, 1933, p. 288. 

6. “The Monaural Threshold, Effect of a Subliminal 
Contralateral Stimulus,” J. W. Hughes, Proceed- 
ings of the Royal Society {London), Vol. 124B, 
1938, p. 406. 

7. “Fluctuation of Hearing Threshold,” S. Lifshitz, 
Journal of the Acoustical Society of America, Vol. 
11, 1939, p. 118. 

8. “Piezoelectric Measurements on the Absolute Audi- 
tory Threshold for Bone Conduction,” G. v. Bekesy. 
Akustische Zeitschrift, Vol. 4, 1939, p. 113. 

9. “Theory of Hearing, Sound Reception by Bone 
Conduction,” G. v. Bekesy, Annalen der Physik, 
Vol. 13, 1932, p. 111. 

10. “Bone Conduction Threshold Measurements,” N. A. 
Watson and R. S. Gales, Journal of the Acoustical 
Society of America, Vol. 14, 1943, p. 207. 

11. “Cancellation of Cochlear Response with Air and 
Bone Conducted Sound,” K. Lowy. Journal of the 
Acoustical Society of America, Vol. 13, 1942, p. 156. 

12. “The Course of the Auditory Threshold in the 
Presence of a Tonal Background,” E. G. Wever and 
S. R. Truman, Journal of Experimental Psychology, 
Vol. 11, 1928, p. 98. 

13. “A Space-Time Pattern Theory of Hearing,” Har- 
vey Fletcher, Journal of the Acoustical Society of 
America, Vol. 1, 1930, p. 311. 


266 


BIBLIOGRAPHY 


14. “Mapping the Cochlea,” E. A. Culler, J. Willman, 
and F. A. Mettler, Ame^'ican Journal of Physiology, 
Vol. 119, 1937, p. 292. 

15. “Revised Frequency Map of the Guinea Pig Coch- 
lea,” E. A. Culler, J. D. Coakley, K. Lowy, and N. 
Gross, American Joiirnal of Psychology, Vol. 56, 
1943, p. 475. 

16. “Differential Pitch Sensitivity of the Ear,” E. G. 
Shower and R. Biddulph, Journal of the Acoustical 
Society of America, Vol. 3, 1931, p. 275. 

17. Hearing, S. S. Stevens and H. Davis, John Wiley 
and Sons, New York, 1938. 

18. “Differential Intensity Sensitivity of the Ear for 
Pure Tones,” R. R. Riesz, Physical Review, Vol. 31, 
1928, p. 867. 

19. “Influence of Experimental Technique on the Meas- 
urement of Differential Intensity Sensitivity of the 
Ear,” H. C. Montgomery. Journal of the Acoustical 
Society of America, Vol. 7, 1935, p. 39. 

20. “Auditory Patterns,” Harvey Fletcher. Reviews of 
Modern Physics, Vol. 12, 1940, p. 47. 

21. “The Mechanism of Hearing,” Harvey Fletcher. 
Proceedings of the National Academy of Sciences, 
Vol. 24, 1938, p. 265. 

22. “Pitch, Quality, and Loudness of Musical Tones,” 
Harvey Fletcher, American Journal of Physics, Vol. 
14, 1946, p. 215. 

Chapter 3 

1. Electrical Oscillatioyis and Waves, G. W. Pierce, 
McGraw-Hill Book Company, New York, 1920, 
p. 320. 

2. Explosions, Propagation in Linear Case (Memoran- 
dum), Harry Nyquist, NDRC, Oct. 31, 1941. 

Div. 6-510.12-M2 

3. Binaural Listening System, Donald P. Loye, NDRC 

6.1-sr20-565, Interim Report P12/145, CUDWR- 
NLL, Jan. 12, 1943. Div. 6-621-Ml 

4. Microstructure of Sea Water, Notes on Measuring 
Temperature Differences (Memorandum), Harry 
Nyquist, C4-NDRC-045, Mar. 12, 1942. 

Div. 6-520.1-M5 

5. Subaqueous Listening, Directivity of a Pair of 

Rings (Memorandum), Harry Nyquist, C4-NDRC- 
064, Apr. 2, 1942. Div. 6-560.2-M2 

6. Noise Trials of H. M. Submarines at Lochgoilhead, 
H. M. Sm. Tempest, N86, O. R. P. J ASTRZAB {late 
American), OSRD W-233-1, Report AL/N.1/D.225, 
Admiralty Research Laboratory, Teddington, Mid- 
dlesex, England, Apr. 2, 1942. 


7. Hydrophone Listening Tests (Technical Memoran- 

dum), Donald P. Loye and Russell O. Hanson, 
NDRC C4-sr20-090, Report G12/2623, CUDWR- 
NLL, Apr. 20, 1942. Div. 6-625.2-M3 

8. Directivity with Two Microphones (Memorandum), 
Harry Nyquist, C4-NDRC-071, June 12, 1942. 

Div. 6-560.2-M4 

9. Sea Backgroimd Noise, OSRD WA-362.5, Report 
ARL/R4/D.225, Admiralty Research Laboratory, 
Teddington, Middlesex, England, July 1942. 

10. Directivity with Two Microphones, Addition vs. 
Multiplication of Outputs (Memorandum), Harry 
Nyquist, C4-NDRC-073, July 1, 1942. 

Div. 6-560.2-M5 

11. Binaural Phenomena, Ralph C. Maninger, Internal 
Memorandum P12/4089, CUDWR-NLL, Sept. 29, 

1942. Div. 6-560. 1-M2 

12. Binaural Listening System, Tests on Amada, Sep- 

tember 15, 19Jf2, J. W. Horton, Donald P. Loye, 
Internal Memorandum P12/4021, CUDWR-NLL, 
Sept. 18, 1942. Div. 6-560.22-Ml 

13. Binaural Tests on the AMADA, September 2U, 
19 U2, Donald A. Proudfoot, Internal Memorandum 
P12/4090, CUDWR-NLL, Oct. 1, 1942. 

Div. 6-560.22-M2 

14. Effects of Airplane Noise on Listening with Head- 

phones, Experiments Conducted at the Psycho- 
Acoustic Laboratory, Harvard University, Septem- 
ber 27 to 29, 19^2 and November 6 to 8, 19Jt.2, Wil- 
liam B. Snow and William D. Neff, OEMsr-20, 
NDRC 6.1-sr20-550, Navy Projects NS-106 and 
NS-198, Memorandum D16/D34/107, CUDWR- 
NLL, Jan. 25, 1943. Div. 6-580.31-Ml 

15. The Use of the CK Tube as an Anti-Submarine 
Listening Device, Irving Langmuir and E. F. Hen- 
nelly, General Electric Company, Feb. 15, 1943. 

Div. 6-622.1-Ml 

16. Survey of Underwater Sound, Introduction, Vern 
0. Knudsen, R. S. Alford, and J. W. Emling, 6.1- 
NDRC-729, Report 1, Feb. 26, 1943. Div. 6-580-Ml 

17. Measurements and Analysis of Sound Pressures of 
Tojyedoes in Range 40 cps to 128 kc/s, A. B. Wood, 
OSRD WA-497-35, M/S Summary 6127/42, Mine 
Design Department, Leigh Park House, Havant 
Hunts, London, Eng., Mar. 10, 1943. Div. 6-913-Ml 

18. Reduction of Range of Detectability of Submarines 
by Reason of Increase in the Depth of Submergence, 
OSRD WA-897-25A, Report ARL/95.14/R.1, Ad- 
miralty Research Laboratory, Teddington, Middle- 
sex, Eng., August 1943. 

19. Binaural Non-Rotating Directional Radio Sonic- 
Buoy System, Elliott J. Lawton, OSRD 1878, NDRC 
6.1-sr323-1108, General Electric Company, Aug. 16, 

1943. Div. 6-624.2-M2 




BIBLIOGRAPHY 


267 


20. SubmaHne Noise and Perforjuance of JP-1 Sonic 

Listening Equipment Aboard the USS Corvina, 
SS226, [from] Septeynber 13 [to] ^4, 19^3, H. C. 
Williams, Navy Projects NS-113, NS-337, Mem- 
orandum for File D24/526, CUDWR-NLL, Sept. 17, 
1943. Div. 6-580.1-Ml 

21. Transmissioyi of Noise Through JP-1 Training 
Gear, Hollie C. Williams, Navy Projects NS-113 
and NS-337, Memorandum for File D24/550, 
CUDWR-NLL, Oct. 14, 1943. Div. 6-623.1-M2 

22. Submarine Listening Systems, Report of Coyi- 
ference September 29, 1943, C. R. Sawyer, Navy 
Projects NS-113 and NS-337, Memorandum for 
File D24/P30/560, CUDWR-NLL, Oct. 26, 1943. 

Div. 6-623. 1-M3 

23. Survey of Uyiderwater Sound, Sounds froyn Sub- 

marines, Vern O. Knudsen, R. S. Alford, and J. W. 
Emling", NDRC 6.1-NDRC-1306, Report 2, Dec. 31, 
1943. Div. 6-580.1-M2 

24. Key West Ty'ials, January 10 [to] 16, 1944, W. L. 

Clearwaters and P. E. Fish, Navy Projects NS-106 
and NS-198, Memorandum for File D16/778, 
CUDWR-NLL, Mar. 2, 1944. Div. 6-580.1-M3 

25. Expendable Radio-Sono Buoy Listening Ranges, 
Second Key West Trials with USS Pintado, March 
13 [to] 19, 1944, American Fleet-Type Submarine, 
P. E. Fish, Navy Projects NS-106 and NS-198, 
Memorandum for File D16/843, CUDWR-NLL, 
Mar. 29, 1944. 

26. Auxiliary and Undey'way Tests on USS Gabilan, 
SS-252, Donald P. Loye and Malcolm T. Rodger, 
Navy Project NS-212, Memorandum for File 
D52/866, CUDWR-NLL, Apr. 10, 1944. 

Div. 6-641.31-Ml 

27. Oscillograyns of 2li-kc Noise Produced by a De- 
stroyer, George E. Duvall, Navy Project NS-140, 
Internal Report A2, UCDWR, May 1, 1944. 

Div. 6-580.2-M2 

28. Underwater Sound Measurements on Aircraft Car- 
y'iers. Navy Project NO-163. Memorandum for File 
M-212, Listening Section, UCDWR, May 15, 1944. 

Div. 6-580.2-M3 

29. Sound Output Measurements on the USS Bluegill 

at 62, 150, and 250-Foot Keel Depths, NDRC 6.1- 
srl046-1050. Navy Project NS-164, MIT-USL, May 
15, 1944. Div. 6-580.1-M6 

30. Progress Report on Listening Systems for Patrol 

Craft, OSRD 1043, NDRC C4-sr692-531, BTL, Nov. 
12, 1942. Div. 6-622.1-M5 

31. A Study of Binaural Perception of the Direction of 

a Sound Source, Irving Langmuir, V. J. Schaefer, 
C. V. Ferguson, and E. F. Hennelly, OSRD 4079, 
NDRC 6.1-sr323-1840, General Electric Company, 
June 30, 1944. Div. 6-560.1-M5 


32. Survey of Underwater Soimd, Aynbient Noise, Vern 

0, Knudsen, R. S. Alford, and J. W. Emling, 6.1- 
NDRC-1848, Report 3, Sept. 26, 1944. 

Div. 6-580.33-M2 

33. Directivity at Low Sonic Frequencies, Walter F. 
Graham and Ralph C. Maninger, Memorandum for 
File P33/1067, CUDWR-NLL, Nov. 8, 1944. 

Div. 6-612.21-M21 

34. Predictioyi of Sonic and Supersonic Listeyiing 

Ranges, OSRD 4761, NDRC 6.1-srll31-1884, Navy 
Project NS-140, Sonar Analysis Section, CUDWR- 
SSG, December 1944. Div. 6-570. 1-M6 

35. Sound Cavitation Tests on USS Springer, SS414, 
Navy Project NS-140, Memorandum for File M-283, 
Listening Section, UCDWR, Dec. 20, 1944. 

Div. 6-580.1-M9 

36. Aynbient Noise Survey, Miami Area and the East 
Coast of the United States, Henry B. Hoff, Donald 
L. Cole, and Robert A. Wagner, NDRC 6.1srll28- 
1944, Navy Project NO-163, Completion Report 
D46A/1215, CUDWR-NLL, May 5, 1945. 

Div. 6-580.33-M3 

Chapter 4 

1. An Investigation of the Audibility of Underwater 
Noises, W. F. Higgins, OSRD WA-317-18, Report 

1, Physics Division, National Physical Laboratory, 
Teddington, Eng., July 17, 1942. Div. 6-560. 21-Ml 

2. An Investigation of the Audibility of Underwater 
Noises, W. F. Higgins, OSRD WA-317-19, Report 

2, Physics Division, National Physical Laboratory, 
Teddington, Eng., July 24, 1942. Div. 6-560. 21-M2 

3. An Investigation of the Audibility of Underwater 
Noises, C. G. Darwin, OSRD 2125-6B, Report 3, 
Physics Division, National Physical Laboratory, 
Teddington, Eng., Apr. 28, 1944. Div. 6-560.21-M3 

4. Submay'ine Listening Range at Loch Goil, Rodney 
F. Simons, OSRD WA-1730-8D, Technical Memo- 
randum, OSRD, London, Eng., Mar. 8, 1944. 

Div. 6-580.1-M4 

5. United States Fleet Anti-Subynarine and Escort 
of Convoy Instructions, Fleet Tactical Preparations 
Bulletin 223, Office of Commander in Chief, U. S. 
Fleet, Navy Dept., 1944. 

6. Masking Experiments ([Part] I), NDRC 6.1-sr30- 
1751, Navy Projects NS-163 and NS-164, Report 
U-229, Listening Section, UCDWR, June 28, 1944. 

Div. 6-560.21-M4 

7. Masking Experhnents ([Part] II) , NDRC 6.1-sr30- 
1757, Navy Projects NS-163 and NS-164, Report 
U-258, Listening Section, UCDWR, Sept. 15, 1944. 

Div. 6-560.21-M6 



268 


BIBLIOGRAPHY 


8. Underwater Sound Measurements on Aircraft Car- 
riers, Service Project NO-163, Report M-212, Listen- 
ing Section, UCDWR, May 15, 1944. 

Div. 6-580.2-M3 

9. Underwater Sound Output [of] USS Spot, SSAIS, 
NObs-2074, Problem 2G, Report M-296, Listening 
Section, UCDWR, Mar. 1, 1945. Div. 6-580.1-M10 

10. Underwater Sound Output of USS Tinosa, SS283, 

Problem 2G, Report M-303, Listening Section, 
UCDWR, Mar. 5, 1945. Div. 6-580.1-Mll 

11. Survey of Underwater Sound, Sounds from Surface 

Ships, M. T. Dow, J. W. Emling, and Vern 0. 
Knudsen, OSRD 5424, NDRC 6.1-NDRC-2124, Re- 
port 4, June 15, 1945. Div. 6-580.2-M7 

12. “Influence of Experimental Technique on the Meas- 
urement of Differential Intensity Sensitivity of the 
Ear,” H. C. Montgomery, Journal of the Acoustical 
Society of America, Vol. 7, 1935, p. 39. 

13. Listening Systems for Patrol Craft (Final Report), 
NDRC 6.1-sr692-1698, BTL, Dec. 1, 1944. 

14. Interval Tests, H. L. Bumbaugh, Memorandum for 
File D13/249, CUDWR-NLL, Apr. 8, 1943. 

Div. 6-560.1-M3 

15. Peak Recognition Tests, Donald L. Cole and Syl- 
vester J. Haefner, Memorandum for File P33/1053, 
CUDWR-NLL, Aug. 14, 1944. Div. 6-560.21-M5 

16. Airplane Detection by Submarines, Edward Ger- 

juoy. Memorandum for File P33/897, CUDWR- 
NLL, May 10, 1944. Div. 6-580.31-M2 

17. Visual Detector of Torpedoes, R. F. West and R. S. 
Rae, OSRD WA-506-17, Internal Report 110, HMA/ 
SEE, Fairlie Laboratory, Eng., Dec. 12, 1942. 

18. Report of Conference [Held at New London on] 
March 10, 19 US, on Listening Techniques, William 
B. Snow, Report Gl/242, CUDWR-NLL, Mar. 29, 

1943. Div. 6-621-M2 

19. Phase Actuated Locator (Final Report), Reginald 

L. Jones, OSRD 1897, NDRC 6.1-sr695-997, BTL, 

Aug. 30, 1943. Div. 6-622.1-M2 

20. Response Characteristics of Interphone Equipment 
(Revision IV to be inserted in OSRD 687), Francis 

M. Wiener, H. Wayne Rudmose, and others, OSRD 

3105, OEMsr-658, Cruft Laboratory, Harvard Uni- 
versity, Jan. 1, 1944. Div. 17-436.321-M3 

21. Headphone Comparison Tests, Merritt B. Jones and 
William B. Snow, Memorandum for File G27 /952, 
CUDWR-NLL, June 1, 1944. Div. 6-624.12-M5 

22. Analyses of Reduction Gear Noise in Water [on 
the] USS Blackfin, SS322, Edwin E. Teal and 
Robert W. Pratt, Navy Project NS-212, Memoran- 
dum for File D53/1042, CUDWR-NLL, July 24, 

1944. Div. 6-580.1-M7 


23. Permofiux Headset for JP Series Equipment, Wil- 
liam B. Snow, Navy Project NS-113, Memorandum 
for File D24/1128, CUDWR-NLL, Sept. 14, 1944. 

Div. 6-623.1-M6 

24. Frequency Weighting in Ship Sound Measurements, 

An Audibility Meter, R. S. Gales, L. S. Goldberg, 
and A. M. Small, Report M-399, UCDWR, Mar. 19, 
1946. Div. 6-560.21-M7 

25. Masking of Submarine Noise by a Sound Composed 

of a Finite Number of Single Frequency Tones, Ed- 
ward Gerjuoy, Report 09.421 (SAG-32), CUDWR- 
SAG, Apr. 28, 1945. Div. 6-560.2-M6 

26. Underwater Telephony, J. W. Horton, OSRD 5183, 

NDRC 6.1-srll28-2211, Navy Project NS-248, Com- 
pletion Report D56/1415, CUDWR-NLL, May 15, 

1945. Div. 6-623.42-M5 

27. An Experimental Study of Masking by a Line 
Spectrum, Problem 2M2, Report M-314, NObs- 
2074, Sonar Data Division, UCDWR, June 7, 1945. 

Div. 6-560.2-M7 

28. Sonic Listening Aboard Submarines, OSRD 5311, 
NDRC 6.1-srll31-1885, Navy Project NS-140, Sonar 
Analysis Section, CUDWR-SSG, February 1945. 

Div. 6-623.1-M8 

Chapter 5 

1. Listening Tests on May lU, 19U3, of Hydrophone for 
Directional Sonic Buoy, Ralph C. Maninger, Service 
Projects NS-330 and AC 55, Memorandum for File 
D34/349, CUDWR-NLL, May 19, 1943. 

Div. 6-624.21-Ml 

2. Comparative Tests on Submarine and Surface Craft 

Listening Equipments, Donald P. Loye and Ralph 
C. Maninger, NDRC 6.1-sr20-1020, Navy Project 
NS-113, Report D24/D38/391, CUDWR-NLL, Sept. 
10, 1943. Div. 6-623-M4 

3. Bearing Accuracy of 3-ft. and 1-ft. Straight Mag- 

netostriction Hydrophones, Ralph C. Maninger, 
Memorandum for File D17/543, CUDWR-NLL, Oct. 
9, 1943. Div. 6-612.62-M17 

4. Comparative Tests on 3-ft., U-ft., and 5-ft. Hydro- 
phones, Ralph C. Maninger, Memorandum for File 
P33/949, CUDWR-NLL, June 12, 1944. 

Div. 6-612.62-M27 

5. Tests of JP-1 Equipment with 3-ft., U-ft., and 5-ft. 

Hydrophones and QB Sound Gear, W. F. Graham, 
Memorandum for File P33/1031, CUDWR-NLL, 
July 13, 1944. Div. 6-623.1-M5 

6. Comparative Field Tests of Underwater Listening 

Equipment Installed on the Elcobel, Walter F. 
Graham and Ralph C. Maninger, OSRD 4391, 
NDRC 6.1-srll28-1569, Report P33/862, CUDWR- 
NLL, Sept. 30, 1944. Div. 6-622.1-M3 


BIBLIOGRAPHY 


269 


7. Tests of an Improved JP-1 Type Hydrophone on 

the USS Blueback, Arthur L. Thuras, Navy Project 
NS-113, Memorandum for File P33/1161, CUDWR- 
NLL, Oct. 3, 1944. Div. 6-612.62-M38 

8. Experimental Investigation of Factors Involved in 

So7iic Listenmg, Ralph C. Maninger, NDRC 6.1- 
srll28-1932, Report P33/1319, CUDWR-NLL, Feb. 
28, 1945. Div. 6-621-M7 

9. Equipment Developed and Used on the AMADA 

for Undei'water Sound Investigations, Walter F. 
Graham, Memorandum for File P33/1379, CUDWR- 
NLL, Feb. 28, 1945. Div. 6-621-M6 

10. Listening Tests of September lU, 1943, on Direc- 
tional vs. Semi-N ondirectional Hydrophone, Ralph 
C. Maninger, Memorandum for File Gl/494, 
CUDWR-NLL, Sept. 28, 1943. 

11. Fundamental Listening Studies at the New London 

Laboratory, Ralph C. Maninger, NDRC 6.1-srll28- 
2210, Summary Report P33/1409, CUDWR-NLL, 
May 30, 1945. Div. 6-621-M8 

12. Prediction of Sonic and Supersonic Listening 
Ranges, OSRD 4761, NDRC 6.1-srll31-1884, Navy 
Project NS-140, CUDWR-SAG, December 1944. 

Div. 6-570.1-M6 

Chapter 7 

1. Frequency Characteristics of Echoes and Rever- 
beration, W. M. Rayton and Raymond C. Fisher, 
OSRD 4159, NDRC 6.1-sr30-1740, Navy Project 
NS-140, Report U-244, UCDWR, Aug. 9, 1944. 

Div. 6-520.3-M2 

2. Submarine Detection, Directivity Indications 

(Memorandum), Harry Nyquist, NDRC, Oct. 11, 

1941. Div. 6-560.2-Ml 

3. Reverberations in Sea Water, Theoretical Consid- 

erations (Memorandum), Harry Nyquist, NDRC 
C4-NDRC-036, Dec. 19, 1941. Div. 6-520.1-M3 

4. Submarine Detection, Tentative Discussion of 

Reverberations (Memorandum), Harry Nyquist, 
NDRC, Oct. 20, 1941. Div. 6-520.1-M2 

5. Submarine Detection, Air Bubbles in Sea Water 
(Memorandum), Harry Nyquist, NDRC, Oct. 28, 

1941. Div. 6-540.2-Ml 

6. Submarine Detection, Aural vs. Automatic Recep- 

tion (Memorandum), Harry Nyquist, NDRC, Dec. 
1, 1941. Div. 6-560.4-Ml 

7. Submarine Detection, Scheme Employing Fre- 
quency Modulation in Transmitted Signal (Memo- 
randum), Harry Nyquist, C4-NDRC-039, Jan. 5, 

1942. Div. 6-560.3-Ml 

8. Binaural Listening for BDI Asdic Sets, Lt. Com. 
F. Moller, R.Nor.N.V.R., T. M. Frey, and H. J. 
Hawkins, OSRD WA-4372-1K, Internal Report 214, 
HMA/SEE, Fairlie Laboratory, Eng., Mar. 27, 
1945. 


9. Build-up Time of Selective Systems, OSRD W-262- 
15, Internal Report 67, HMA/SEE, Fairlie Labora- 
tory, Eng., Mar. 19, 1942. 

10. Detection of Radio Echoes, Noise Discrimination 

(Memorandum), Harry Nyquist, C4-NDRC-063, 
Mar. 27, 1942. Div. 6-560.31-Ml 

11. Reverberations in Sea Water, Note on Reflections 
at the Surface (Memorandum) Harry Nyquist, 
C4-NDRC-065, Apr. 3, 1942. Div. 6-520.11-Ml 

12. Split Beam Direction Finding, Intei'im Report, 
OSRD WA-107-11, Internal Report 77, HMA/SEE, 
Fairlie Laboratory, Eng., Apr. 22, 1942. 

Div. 6-631.41-M2 

13. Reaction Times to Sight and Sound Under Certain 
Conditions, OSRD WA-171-9, Internal Report 90, 
HMA/SEE, Fairlie Laboratory, Eng., June 1942. 

14. Observations of Echo Signals Obtained Using Vari- 
able Frequency Transmission (Memorandum for 
file), Edwin M. MacMillen, NDRC C4-sr30-208, 
Navy Project NS-140, UCDWR, July 4, 1942. 

Div. 6-510.3-Ml 

15. Echo Ranging at Lower Frequencies, L. M. Danger, 
OSRD 1954, NDRC 6.1-sr30-1114, Navy Project 
NS-140, Interim Report U-109, UCDWR, Sept. 1, 

1943. Div. 6-510.1-M2 

16. Hydrophoiie Effect from Icebergs, G. E. R. Deacon, 
OSRD WA-1101-10, Internal Report 144, HMA/ 
SEE, Fairlie Laboratory, Eng., Sept. 13, 1943. 

17. The Reverbe7'ation Equalizer, George W. Downs, 
Jr., OSRD 3100, NDRC 6.1-sr30-692, Navy Project 
NS-142, Report U-97, UCDWR, Sept. 18, 1943. 

Div. 6-520.2-M2 

18. Echo Detection of Small Targets, H. F. Willis, D. 
W. Boston, W. A. Jones, and R. T. Ackroyd, OSRD 
WA-1154-lb, Internal Report 145, HMA/SEE, 
Fairlie Laboratory, Eng., Sept. 23, 1943. 

Div. 6-633.24-Ml 

19. Submarine Detection, Pinging Patterns (Memo- 
randum), Harry Nyquist, 6.1-NDRC-126, Mar. 28, 

1944. Div. 6-560.3-M3 

20. Echo Ranging from Submarines, Range Indicator 

Requirements, J. Warren Horton, Navy Project 
NS-142, Memorandum for File P29/1076, CUDWR- 
NLL, Aug. 16, 1944. Div. 6-631-M4 

21. Echoes from Swells, George E. Duvall, Navy Proj- 

ect NS-141, Internal Report A43, UCDWR, Oct. 27, 
1944. Div. 6-540-Ml 

22. Echo Doppler Indicator, J. Warren Horton, OSRD 

4511, NDRC 6.1-srll28-1924, Navy Project NS-142, 
Completion Report P36/1262, CUDWR-NLL, Dec. 
15, 1944. Div. 6-631.34-M5 


270 


BIBLIOGRAPHY 


23. Recommendations for Revision of Submarine Echo- 

Ranging System, J. Warren Horton, OSRD 5273, 
NDRC 6.1-srll28-2219, Navy Project NS-142, Sum- 
mary Report P29/1435, CUDWR-NLL, May 24, 
1945. Div. 6-631.5-M6 

24. Mine and Torpedo Detection Equipment, J. Warren 
Horton, OSRD 5552, NDRC 6.1-srll28-2220, Navy 
Project NS-297, Progress Report P66/1436, 
CUDWR-NLL, May 28, 1945. Div. 6-633.21-M2 

25. A New Method of Classifying Bottomed U-Boats 
by Means of Shadows, P. C. Browne, E. Gresty, and 
H. Nodtredt, OSRD WA-4615-9, WA-5057-6, and 
WA-5176-11, Internal Report 220, HMA/SEE, 
Fairlie Laboratory, Eng., June 5, 1945. 

26. Sonar Hut Noise Measurements on a Frigate, 
NObs-2074, Problem 2F, Report M-324, Listening 
Section, UCDWR, June 11, 1945. 

Div. 6-580.31-M3 

Chapter 8 

1. Telegraph Signaling, Aural Reception vs. Other, 

Report of Laboratory Tests, R. S. Alford, OEMsr- 
346, Report 3410-RSA-MS, Case 23 228-15, BTL, 
July 2, 1942. Div. 6-560.31-M2 

2. ‘‘Mathematical Analysis of Random Noise,” S. O. 
Rice, The Bell System Technical Journal, Vol. 23, 
1944, p. 282, and Vol. 24, 1945, p. 46. 

3. Status Report on Task No. 5, Effect of Short Pulse 

Lengths and Receiver Bandwidth on Echo Ranging, 
BTL, July 15, 1944. Div. 6-632.03-M5 

4. Frequency Modulation, Saw Tooth Modulation 
Equivalent to Succession of Single Frequency 
Transients with Rounded Envelopes (Memorandum 
January 5, 1942), Kenneth W. Pfleger, NDRC, File 
36680-3, BTL, Revised Mar. 19, 1942. 

Div. 6-635.1-Ml 

5. Selectivity in Asdic Reception, OSRD WA-362-9, 

Internal Report 109, HMA/SEE, Fairlie Labora- 
tory, Eng., Nov. 9, 1942. Div. 6-560. 3-M2 

6. Masking Effect of Water Noise on Short Pulses, 

Raymond C. Fisher, NDRC 6.1-sr30-1538, Navy 
Project NS-221, Report S-239, UCDWR, July 25, 
1944. Div. 6-634.2-Ml 

7. The Detectability of Repeated Pulses, Raymond C. 

Fisher and Carl F. Eckart, Navy Project NS-221, 
Memorandum for File M-261, UCDWR, Sept. 27, 
1944. Div. 6-560.3-M5 

8. Submarine Detection, Directivity Indications 

(Memorandum), Harry Nyquist, NDRC, Oct. 11, 

1941. Div. 6-560.2-Ml 

9. Reverberations in Sea Water, Theoretical Consid- 

erations (Memorandum), Harry Nyquist, C4- 
NDRC-036, NDRC, Oct. 19, 1941. Div. 6-520.1-M3 


10. Submarine Detection, Aural vs. Automatic Recep- 

tion (Memorandum), Harry Nyquist, NDRC, Dec. 
1, 1941. Div. 6-560.4-Ml 

11. Telegraph Theory, Aural Reception vs. Other 

(Memorandum), Harry Nyquist, NDRC, Dec. 11, 
1941. Div. 6-560. 4-M2 

12. Sensitivity of Recorder Paper, OSRD WA-254-14, 
Internal Report 92, HMA/SEE, Fairlie Laboratory, 
Eng., Aug. 1, 1942. 

13. Alternating Current Sensitivity of Recorder Paper, 
OSRD WA-317-26, Internal Report 99, HMA/SEE, 
Fairlie Laboratory, Eng., September 1942. 

14. Split Beam Direction Finding, R. D. Keynes and 
R. B. Serle, OSRD WA-647-4, Internal Report 91, 
HMA/SEE, Fairlie Laboratory, Eng., Apr. 16, 
1943. 

15. Eosin Impregnated Recorder Paper, E. A. Alex- 
ander and J. Hawkins, OSRD WA-716-35, Internal 
Report 133, HMA/SEE, Fairlie Laboratory, Eng., 
May 20, 1943. 

16. Standard Method of Testing Recoy'der Paper, J. 
Hawkins, OSRD WA-716-36, Internal Report 134, 
HMA/SEE, Fairlie Laboratory, Eng., May 29, 

1943. 

17. Silent Fathometers, General Discussion (Memoran- 
dum), Harry Nyquist, 6.1-NDRC-123. Oct. 20, 1943. 

Div. 6-634.1-M4 

18. BDI Sample Traces (External Memorandum), 
OSRD 3402, NDRC 6.1-sr287-1441, HUSL, Feb. 21, 

1944. Div. 6-631.411-M8 

19. Sea Tests of Overhearing of the Secure Echo- 

Sounding Equipment, SESE Model 2l Aboard the 
Submarine SS^l, Spadefish, David H. Ransom and 
Raymond C. Fisher, NDRC 6.1-sr30-1843, Navy 
Project NS-221, Memorandum SM-251, UCDWR, 
Aug. 22, 1944. Div. 6-634.2-M2 

20. Single Tone Carrier Telegraphy, Comparison of 
Masking Effect of Thermal Noise vs. Reverbera- 
tions, Kenneth W. Pfleger, 6.1-NDRC-1482, Report 
210-KWP-QY, Aug. 28, 1944. Div. 6-560.32-M4 

Chapter 9 

1. Laboratory Tests of the Masking of Echoes by 
Reverberation, R. S. Alford and W. E. Reid, NDRC 
6.1-sr346-2126, BTL, June 8, 1945. 

Div. 6-560.32-M6 

2. Reverberation Studies at 2U kc, OSRD 1098, NDRC 
C4-sr30-401, Navy Project NS-140, Report U-7, 
Reverberation Group, UCDWR, Nov. 23, 1942. 

Div. 6-520-M2 

3. Status Report on Task No. 5. Effect of Short 
Pulse Lengths and Receiver Bandwidth on Echo 
Ranging, BTL, July 15, 1944. Div. 6-632.03-M5 


BIBLIOGRAPHY 


271 


4. Computed Maximum Echo and Detection Ranges 
/or Submarine Echo-Ranging Gear, William B. 
Snow and Edward Gerjuoy, NDRC 6.1-srll31, 
1128-1688, Navy Project NS-140, Sonar Analysis 
Section, CUDWR-NLL, July 1944. Div. 6-570-M2 

5. Echo Recognition Group Training, as developed for 

use with the echo recognition Monitor Recorder 
Model 2, Problem 5H, Report U-325, UCDWR, July 
10, 1945. Div. 6-321.4-Ml 

Chapter 10 

1. The Effect of Doppler on Echo Detection, OSRD 
WA-262-10, Internal Report 81, HMA/SEE, Fairlie 
Laboratory, Eng., May 1942. 

2. Frequency Sensitivity Curves for Type W-621 
Headphones, OSRD W-282-15, Internal Report 89, 
HMA/SEE, Fairlie Laboratory, Eng., May 21, 1942. 

Div. 6-560.2-M3 

3. Comparison of Loudspeaker and Telephones for 
Detectability of Weak Echoes, H. N. McNair and 
R. Hall, OSRD WA-2246-5A, Internal Report 178, 
HMA/SEE, Fairlie Laboratory, Eng., May 8, 1944. 

Div. 6-560.3-M4 

4. Comparison of Loudspeaker and Telephone for 

Recognition of Doppler, OSRD WA-3065-8, Internal 
Report 199, HMA/SEE, Fairlie Laboratory, Eng., 
Oct. 17, 1944. Div. 6-560.32-M5 

5. Interim Report on Electronic Own Doppler Nul- 
lifier {Mark II), A. W. Nolle and W. A. Felsing, 
NDRC 6.1-sr287-719, HUSL, Mar. 24, 1943. 

Div. 6-631.31-M5 

6. Automatic Gain Control in Echo-Ranging Systems 
(Memorandum for file), F. V. Hunt, NDRC 6.1- 
sr287-764, HUSL, Apr. 13, 1943. Div. 6-631.13-Ml 

7. Description of Devices to be Demonstrated on USS 
Semmes and USS Galaxy (Memorandum), NDRC 
6.1-sr287-776, HUSL, May 22, 1943. 

Div. 6-631-Ml 

8. Automatic Gain Control in Echo Ranging Systems, 

J. Anderson, OSRD WA-854-15-a, Report D-4330, 
HMA/SEE, Fairlie Laboratory, Eng., July 14, 
1943. Div. 6-560.32-M2 

9. Time-Varied Gain for Sonar Equipment (Comple- 

tion Report), OSRD 5340, NDRC 6.1-sr287-2076, 
HUSL, June 15, 1945. Div. 6-631.11-M3 


10. Reverberation Controlled Gain for Sonar Equip- 
ment (Completion Report), F. V. Hunt, OSRD 
5415, NDRC 6.1-sr287-2079, HUSL, July 15, 1945. 

Div. 6-631. 12-M5 

11. Sonar Doppler Applications, F. V. Hunt, OSRD 
6558, NDRC 6.1-sr287-2069, HUSL, Nov. 15, 1945. 

( Div. 6-631.3-M5 

12. Frequency-Sensitivity Curves for Brown A Type 
Headphones, T. Emmerson, OSRD WA-394-21, In- 
ternal Report 114, HMA/SEE, Fairlie Laboratory, 
Eng., December 1942. 

13. The Doppler Doubler and Square-Law Amplifica- 
tion (Memorandum for File), W. A. Meyers, Navy 
Project NS-142, Report M48, UCDWR, Apr. 1, 
1943. 

14. Notes on Doppler Enhancer and Indicators, OSRD 
WA-389-31e, May 15, 1943. 

15. Tone Duration as a Factor in Pitch DiscHmination, 

E. G. Wever, NDRC 6.1-sr30-1465, Navy Project 
NS-97, Memorandum for File M-179, UCDWR, Feb. 
16, 1944. Div. 6-560.1-M4 

16. The D-Series of Doppler Drills and Tests, A Report 

on the Psychological Standards of Auditory Dis- 
crimination, Adelbert Ford, OSRD 3691, NDRC 
6.1-sr30-1505, Navy Project NS-97, Report U-206, 
UCDWR, March 1944. Div. 6-560.32-M3 

17. A Survey of U003 Audiograms in Relation to the 

Performance of Sonar Operations at the West 
Coast Sound School, Adelbert Ford, Navy Project 
NS-97, UCDWR, April 1944. Div. 6-311-MlO 

18. Echo Doppler Indicator, J. Warren Horton, OSRD 

4511, NDRC 6.1-srll28-1924, Navy Project NS-142, 
Completion Report P36/1262, CUDWR-NLL, Dec. 
15, 1944. Div. 6-631.34-M5 

19. Coinparison of Echo Recognition at 800 and 500 
Cycles, Adelbert Ford and L. J. Cronbach, Report 
M-312, UCDWR, Apr. 30, 1945. 

20. Doppler Judgment at Low Beat-Frequency Oscilla- 
tor Settings, Adelbert Ford, L. J. Cronbach, and 
D. F. Lovell, Report M-347, UCDWR, Aug. 10, 1945. 

21. Studies of the Recognition of Submarine Echoes, 
Report M-431, Sonar Data Division, UCDWR. 



CONTRACT NUMBERS, CONTRACTORS, AND SUBJECT OF CONTRACT 


Contract Number 


Name and Address of Contractor 


Subject 


OEMsr-20 


OEMsr-1128 


The Trustees of Columbia University 
in the City of New York 
New York, N. Y. 


The Trustees of Columbia University 
in the City of New York 
New York, N. Y. 


Studies and experimental in- 
vestigations in connection 
with and for the develop- 
ment of equipment and 
methods pertaining to sub- 
marine warfare. 

Conduct studies and experi- 
mental investigations in 
connection with and for the 
development of equipment 
and methods involved in 
submarine and subsurface 
warfare. 


OEMsr-1131 


OEMsr-30 


OEMsr-346 


OEMsr-692 


The Trustees of Columbia University 
in the City of New York 
New York, N. Y. 


The Regents of the University of California 
Berkeley, Calif. 


Western Electric Company, Inc. 
New York, N. Y. 


Western Electric Company, Inc. 
New York, N. Y. 


Conduct studies and investi- 
gations in connection with 
the evaluation of the ap- 
plicability of data, meth- 
ods, devices, and systems 
pertaining to submarine 
and subsurface warfare. 

Maintain and operate certain 
laboratories and conduct 
studies and experimental 
investigations in connection 
with submarine and sub- 
surface warfare. 

Studies and experimental in- 
vestigations in connection 
with submarine and sub- 
surface warfare. 

Conduct studies and experi- 
mental investigations in 
connection with the devel- 
opment of listening and de- 
tecting systems suitable for 
surface craft and for sub- 
marines. 


I 



272 


INDEX 


The subject indexes of all STR volumes are combined in a master index printed in a separate volume. For 
access to the index volume consult the Army or Navy Agency listed on the reverse of the half-title page. 


Absolute thresholds, 12-17 

audibility threshold, 12-16, 59-61 
bone conduction threshold, 16 
definition, 12 
pain threshold, 17 
Acoustic fathometers, 200 
Acoustic leakage around headphone 
caps, 248 

Acoustic radiation from a surface 
ship or submarine 
see Target sounds 
Acoustic shadows, 42 
Action potentials of the auditory 
nerve, 9 

Adjacent masking, 18-20, 35, 244, 
258 

Air particle displacement at sound 
threshold, 14 

Airplane noise, 37, 46, 153 

heterodyning for improved de- 
tection, 132-133 

AM pulses masked by noise, 182- 
185, 207 

comparison with FM signals, 
182-185 

effect of double pulse, 182 
effect of pulse repetition fre- 
quency, 182 
Ambient noise, 46-48 
in tropical waters, 48 
masking efficiency, 187 
shrimp crackle, 48 
Angular discrimination of a hydro- 
phone, 92, 135, 137-140 
Animal sensitivity to sound, meth- 
ods of testing, 8-9 
Attenuation of sound, selective, 45, 
56 

Audibility of noise-masked echoes 
see Noise masking of echoes 
Audibility of reverberation-masked 
echoes 

see Reverberation masking of 
echoes with doppler; Rever- 
beration masking of echoes 
without doppler 
Audibility of target sounds 

see Masking of target sounds, 
field tests; Masking of tar- 
get sounds, laboratory tests 
Audibility threshold, 12-16 

altitude and acceleration effects, 
12 

British masking tests, 59-61 


definition, 12 
diffraction effects, 14 
echo effects, 15 

effect of ossicle stiffness, 13 < 

effect of round v/indow mass, 13 
for a 1 kc tone, 16 
free field threshold, 15 
frequency effect, 13 
headphone threshold, 14, 15 
maximum air particle displace- 
ment, 14 

monaural and binaural thresh- 
olds, 15 

observer variability, 15 
thermal noise limit in the ear, 
14 

Audible sound, definition, 52 
Auditory acuity test (Seashore 
test), 222 
Auditory motion 

see also Time-amplitude pat- 
terns 

amplitude modulation of pro- 
peller sounds, 116 
audibility of tones in the pres- 
ence of distributed back- 
ground, 76 

bearing errors, 138-139 
critical bands, 36, 37 
effectivness, 127 
fluctuating sounds, 95 
frequency limen, 30 
hydrophone directivity, 91, 134- 
140, 152 

hydrophone discrimination, 50 
hydrophone sweep, 151, 152 
intensity level, 110 
intensity limen, 34 
interval tests of masking, 128 
loudness discrimination, 31, 33- 
34 

machinery sounds, 42 
minimal intensity increments, 
106 

modulated sounds, 154 
pitch discrimination, 28-31 
propeller beats, 40 
signal modulation, 150 
spectrum of a noise source, 41 
system tuning. 111 
Auditory nerve, 2, 7 

see also Ear, response character- 
istics; Ear structure 
action potentials, 9 


Auditory strings, 3-6 
effective mass, 4 

formula for mass per unit length, 
4 

length, 4 

mass loading per string, 6 
position upon basilar membrane, 
4 

tension from spiral ligament, 4 
Aural harmonics, 24-28 

analysis by method of beats, 26 
aural critical bands, 35 
sensation levels, 26 
Automatic signal selector, 93 

Background sound 

see also Noise background; Pro- 
peller sounds 
characteristics, 260 
definition, 17 

effect on recognition differen- 
tial, 102 

in field tests of masking, 141-144 
masking efficiency of rough and 
smooth noise, 207-208 
modulation, 154 
reverberation, 164-169 
reverberation level, 231, 249 
reverberation recordings, 218- 
219 

reverberation spectrum, 237 
spectrum, 153, 260 
volume control of reverberation, 
240 

well-balanced, 60 
Baffie, acoustic, 49, 50 
Band width 

critical, 36, 125, 172, 184 
effect on signal audibility, 114 
effect on sound fluctuation, 110, 
111 

noise masking of echoes, 188 
Band-pass Alters, 36, 110, 213 
Basilar membrane 

see also Stimulation pattern on 
basilar membrane 
auditory strings, 4 
Corti arches, 7 
position in ear, 3 
position of maximum stimulation 
vs. frequency, 28 
resonance, 3 
tension, 5-6 


274 


. INDEX 


Bass-boost filter, 144 
Beam-aspect ship echoes, 162, 196 
Bearing accuracy, definition, 137 
Bearing errors, 48, 136-139 
Beats, 20-28 

aural harmonics, 24-28 
causing threshold shifts, 20 
effect on basilar membrane, 20, 
23 

intertone, 23 

sensations produced by various 
beating frequencies, 23 
Bell Telephone Laboratories 
see BTL tests 

Binaural masking effect, 19 
Binaural threshold, 15 
Blade rate modulation, 153 
Bone conduction threshold, 16 
curve, 16 
definition, 12 

method of determining, 16 
Bottom reverberation, ocean, 166 
British tests 

masking of target sounds, 55-92 
noise masking of echoes, 186-188 
radiosonic equipment, 130 
reverberation masking of echoes 
with doppler, 225 
BTL tests 

loudness of short pulses, 32-33 
noise masking of echoes, 170-185 
reverberation masking of echoes 
without doppler, 209-217 

Cavitation noise, 39, 57, 117, 153 
Chirp, 262 

Clean echoes, 162, 225 
Cochlea, ear, 2-3 
cochlear canal, 3 
cochlear duct, 2 
cochlear fluid, 2 
cochlear potentials, 8 
Columbia Univ. Div. of War Re- 
search 

see CUDWR tests 
Complex sounds, 34-36 
definition, 34 
filtering, 35 
loudness, 35 
masking, 35 
Continuous wave pulses 
see CW pulses 
Corti arches, 7, 30 
Critical band width 

see Beats; Pitch discrimination 
Critical band width for pulses 
FM signal detection, 184 
masking of dopplered pulses, 242, 
258 

noise masking of echoes, 171- 
175 


noise masking of single pulses, 
192 

Critical band width for sustained 
tones 

aural critical bands, 35, 36 
critical band rule, 76 
critical band spectra, 94 
masking of pure tones, 122, 125 
CUDWR-NLL tests, masking of 
target sounds, 128-133 
CUDWR-USRL tests, noise mask- 
ing of echoes, 185-186 
CW pulses, 164 

loudness of primaudible pulse in 
absence of masking, 191 
masking of rectangular pulses, 
185 

noise masking, 190-193 
recognition in the presence of 
noise, 182 

CW reverberation, 165-168 

amplitude modulation, 165-166 
decay rate, 166 
doppler shifts, 166 
effect of motion of echo-ranging 
projector, 166 

frequency properties, 166-167 
herterodyne frequency, 167-168 
periodmeter measurements, 167 
tonality, 166 
width of spectrum, 167 

Damping, 4, 132 
Deafness 
causes, 1 

due to missing hair cells, 8 
post-mortem study of deafened 
human ears, 28 

Decay curve for reverberation, 166 
Decay rate of CW reverberation, 
166 

Decibel scale for underwater acous- 
tics, 16 

Deep-sea ambient noise, 46, 117 
Detection ratio for a given signal, 
91 

Diffraction of sound, effect on audi- 
bility threshold, 14 
Directional hydrophones, 37, 50, 88, 
113 

Directivity patterns, 42 
see also Hydrophone directivity 
Disc hydrophone, 135 
Distortion effects 

noise masking of echoes, 198-200 
reverberation masking of echoes 
without doppler, 232-233 
Dome, 46, 135 
Doppler doubler, 243 
Doppler drill for sonar personnel 
selection, 125 


Doppler effect, 164, 234-237 

see also Reverberation masking 
of echoes with doppler 
CW reverberation, 166 
definition, 234 

formula for doppler shift, 235 
magnitude of doppler shift, 236 
own doppler, 235 
sources of doppler shift, 235 
Double pulse, 182 
Down-doppler, 235, 241 
Drum membrane, ear, 1 

Ear, response characteristics, 12- 
36 

absolute thresholds, 12-17 
build-up time, 32-33, 183, 228 
complex sounds, 34-36 
distributed sounds, 36 
for short pulses, 192, 212 
frequency discrimination, 28-31 
headphones, 238-240 
headphone threshold, 14, 15, 60 
loudness, 31-34 
masking of short pulses, 204 
mechanisms reducing amplitude 
of loud sounds, 10 
pitch discrimination, 28-31 
tests of sensitivity, 8-9 
thermal noise in the ear, 14 
tone discrimination, 28-34 
Ear structure, 1-11 
canal, 1, 10 
drum, 10, 11 
inner ear, 2-3 
middle ear, 10-11 
outer ear, 9-10 
resonant mechanism, 3-7 
sensory and nervous structures, 
7-8 

Echo amplitude measurements, 221 
Echo and reverberation recordings, 
215-220 

minimizing the effects of noise, 
217 

phase interference effect, 220 
synchronization of echo and re- 
verberation, 219 
Echo injection, 186 
Echo recognition differentials 
see Recognition differentials 
Echo-ranging gear, 37 
Echo-ranging tests 

see Noise masking of echoes; 
Reverberation masking of 
echoes with doppler; Re- 
verberation masking of 
echoes without doppler 
Echoes, 161-164 

see also Pulses for echo ranging 
amplitude, 162-164, 260 


INDEX 


275 


beam aspect ship echoes, 162, 196 
clean echoes, 162, 225 
distortion by reflection from 
target, 162 
floppier shift, 164 
echo-background mixture, 261 
echo recognition level, 260 
energy in the spectrum, 164 
frequency, 164 
from FM sound, 164 
from large targets, 162 
from small targets, 162 
from submarine and nonsub- 
marine targets, 219 
intensity, 164 
off-beam, 162 
resemblance to pulse, 162 
smear echo, 162 
sound power, 260 
spectrum, 260 
specular reflection, 162 
Echoes, masking 

see Noise masking of echoes; 
Reverberation masking of 
echoes with floppier; Re- 
verberation masking of 
echoes without floppier 
Electrical filters for masking tests, 
85 

Electrical method of testing ear 
sensitivity, 8-9 
Electrical noise, 46, 153 
Electrical potentials in the ear 
action potentials of the auditory 
nerve, 9 

cochlear potentials, 8 
electrophonic effect, 8 
Eustachian tube, ear, 10 
External ear 
see Outer ear 

Fathometers, acoustic, 200 
Fidelity in supersonic recordings, 
118 

Field tests 

masking of target sounds, 134- 
152 

reverberation masking of dop- 
plered pulses, 237-251 
Film recordings of underwater 
sound, 56 

Filters 

background filters, 88 
band-pass, 36, 110, 213 
bass-boost filter, 144 
critical band, 76-78 
effect of removing low frequen- 
cies, 86 

electrical filters, 85 
for a pure tone mixed with dis- 
tributed sound, 41 


for complex sounds, 35 
for simulating effect of hydro- 
phone sensitivity, 88 
high-pass, 50, 85 
improvement in sound detect- 
ability, 85 

need for common filter for signal 
and noise, 205 
notch filter, 249 

octave band-pass, 56, 76-78, 85, 
113 

tests, 85-87, 150-151 
FM pulses 

for echo ranging, 159-160 
resultant beam and off-beam 
echoes, 164 

FM pulses, noise masked, 182-185 
critical band width, 184 
factors affecting detection, 184 
rate of sweep, 184 
recognition differentials, 185 
shapes, 184 

FM reverberation, 168-169 
amplitude modulation, 168 
comparison with CW reverbera- 
tion, 168 
intensity, 168 

loss of floppier discrimination, 
169 

rate of variation, 168 
recognition differentials, 262 
smoothing, 168 
spectrum, 168 
use in mine detection, 169 
Formulas and calculations 

amplitude of echo-ranging pulse, 
157 

bearing error, 137 
corrections for curvature of 
sound field, 140 
critical band width, 36 
floppier shift, 235 
frequency discrimination, 30 
frequency of noise peaks, 180 
hydrophone lobe width, 51 
maximum displacement of air 
particle at threshold inten- 
sity, 14 

octave filters, characteristics, 57 
Rayleigh distribution, 165, 168 
resonant frequency of an audi- 
tory string, 4 
spectrum of a pulse, 157 
spread in reverberation spec- 
trum produced by own-dop- 
pler, 237 

spread of a transition curve, 81 
Free-field threshold, 15, 122 
Frequencies, standard reference 
band, 58, 82 


Frequency discrimination 
see Pitch discrimination 
Frequency effect on audibility 
threshold, 13 

Frequency limen, definition, 29 
Frequency modulated pulses 
see FM pulses 

Frequency stabilizer, 236, 250 
Frequency translation of airplane 
noise, 132-133 

Gain control for reverberation 
masked echoes 

automatic volume control (A VC) , 
240 

reverberation control of gain 
(RCG), 240 

time varied gain (TVG), 240 
Gain settings 

see also Loudness level 
in tactical listening, 117 
level stabilizer, 245 
masking of target sounds, 88 
noise masking of echoes, 174 
Gate circuit, 215, 216 
Glides, FM pulses, 183 

Hair cells in the ear 
description, 7 
effect on hearing, 8 
Hairlets in the ear, 7 
Harmonics, aural, 24-28 

analysis by method of beats, 26 
sensation levels, 26 
Headphone audibility threshold, 14 
difference between headphone 
and free field threshold, 15 
for pure tones, 60 
for reverberation-masked dop- 
plered pulses, 239 
Headphones, 238-240, 250-251 
acoustic leakage around caps, 
248 

dopplered pulse detection, 250- 
251 

factors affecting performance, 
59 

Hearing 

see Ear, response character- 
istics; Ear structure 
Heterodyne frequency, 188 
factors affecting use, 196 
operator fatigue, 193 
reverberation masked echoes 
with floppier, 248-250 
Heterodyned reverberation, 167- 
168 

Heterodyned target sounds, 117- 
122 

Heterodyning airplane noise, 133 
High-pass filters, 50, 85 


276 


INDEX 


Hydrophone, 37 

angular discrimination, 92, 135, 
137-140 
axis, 37 

bearing accuracy, 137-140 
circular disc, 135 
effect of receiver noise discrimi- 
nation, 150 

factors determining performance, 
144, 150 

installations, 46 
JP-1; 134 
on-off effect, 137 
need for discrimination against 
isotropic noise, 150 
piston type, 49 
sweep rate, 136, 137-140 
training a hydrophone, 136, 145 
Hydrophone, directional, 37, 88, 113 
discrimination against nondirec- 
tional noise, 50 
Hydrophone, line, 49 

angular discrimination, 137-140 
bearing accuracy, 137-140 
lobe width, 135 
long line, 138 
short lines, 138 
sweep rate, 137-140 
Hydrophone, tubular, longitudinal 
resonance, 131 
Hydrophone directivity 
advantages, 135 
definition, 37 

discrimination against sea noise, 
134 

effect on signal audibility, 88-92 
factors determining, 48-49 
frequency discrimination, 52 
improvement, 50 
in sonic masking, 134-140 
lobes, 51, 52, 134 
mechanism of, 51 
simulated, 88-92 
Hydrophone effect, 187 

Injected pulses, reverberation mask- 
ing, 211-215 
Inner ear, 1-7 
cochlea, 2 

resonant frequency, 3, 5 
resonant mechanism, 3-7 
Integration of nerve impulses 

see Ear, response characteristics; 
Ear structure 
Intensity limen, 33, 115 
Interference patterns of target 
sounds, 45 

Interfering background 

see Noise background; Reverbera- 
tion background 


JP-1 hydrophone 

filter for amplifier unit, 144 
frequency response, 142 
construction, 134 
directivity, 135 
mounting, 135 
resonance peaks, 131 
sweep modulation, 137 
TTH gear, 135 

Level stabilizers for gain setting, 
245 

Limen, frequency, 29 
Limen, intensity, 33, 115 
Liniinal 

see Thresholds of the ear 
Line hydrophone 

see Hydrophone, line 
Listening 

masking-limited, definition, 52 
threshold-limited, definition, 52 
Listening gear, 37, 43, 48-52 
bearing accuracy, 50 
bearing error, 48 
functions, 48 
hydrophones, 48-49, 50 
range, 48 
sonic, 46, 135 
supersonic, 46, 50, 135 
TTH gear, 135 
Listening tests 
apparatus, 55 
effect of guessing, 79 
effect of observer’s attitude, 79 
effect of observer’s perception, 
memory and judgment, 80 
field tests; see Masking of target 
sounds, field tests 
laboratory measurements ; see 
Masking of target sounds, 
laboratory tests 
operator fatigue, 80 
requirements, 54 

Lobes, hydrophone directivity, 49, 
50 

see also Hydrophone directivity 
Loudness, 31-34 
beats, 20-28 
discrimination, 33-34 
effect of duration of sound, 32 
minimum audible changes, 21, 23, 
52, 53 

intensity limen, 33 
measurement, 31 
of complex sounds, 35, 68, 87-88, 
102, 103 

of pulses, 32-33, 177-182 
of sustained tones, 31 
Loudness level 
definition, 31 
pure tone masking, 245 



reverberation-masked echoes, 226, 
231-232 

spectrum differential, 116-117 
tests of masking, 87-88 
Loudspeaker as a dopplered pulse 
detector, 250-251 

Machinery sounds 

effect of filtering on detection, 86 
modulation, 153 
spectra, 41-42, 57-58 
Magnetostrictive hydrophone JP-1; 
134 

Magnetostrictive projector, 159 
Masked tone, definition, 17 
Masking, 17-28, 52-53 

adjacent and remote masking, 19- 
20, 35, 244, 258 

aural harmonics produced, 24-28 
binaural effect, 19 
by airborne sounds, 16, 46 
by continuous background, 52-53 
by distributed sounds, 36 
by group of tones, 53 
by single tone, 52 ^ 
definition, 17 

effect of auditory nerve tract, 19 
effect of beats, 20-24 
effect of modulation on detection, 
136 

fluctuation with intensity, 21 
frequency effects, 19 
level, 17 

of complex sounds, 35 
of noise pulses by noise, 205 
of steady sounds, 154 
threshold shifts, 17-19 
Masking, subjective reactions 
see Subjective reactions, masking 
tests 

Masking of AM pulses 

see AM pulses masked by noise 
Masking of dopplered echoes 

see Reverberation masking of 
echoes with doppler 
Masking of echoes without doppler 
see Noise masking of echoes; Re- 
verberation masking of 
echoes without doppler 
Masking of FM pulses 

see FM pulses, noise masked 
Masking of propeller sounds 

see Propeller sounds, sonic; Pro- 
peller sounds, supersonic 
Masking of pure tones, 122-125, 245 
audibility of background, 124 
critical-band criterion, 125 
gain setting, 124 
noise backgrounds, 123 
primaudible tone levels, 124 


INDEX 


277 


Masking of target sounds, field tests, 
134-152 

Masking of target sounds, field tests, 
methods 

difficulties, 134, 146 
methods and equipment, 140-141, 
144-146 

presentation methods, 145 

Masking of target sounds, field 
tests, results 
auditory motion, 151-152 
background, 141-144 
effect of joint motion of hydro- 
phone and target, 145 
effect of training hydrophone, 145 
hydrophone directivity, 134-140 
method of determining primau- 
dible signal levels, 145 
observed recognition differentials, 
146-151 

signals and background, 141-144 
variables, 134 

Masking of target sounds, labora- 
tory tests, 54-133 

Masking of target sounds, labora- 
tory tests, methods 
apparatus, 55, 80, 93 
British tests, 55-92 
CUDWR-NLL tests, 128-133 
headphone performance, 59 
increasing signal-level method, 
82-85 

interval tests, 128-130 
learning and training, 93 
method of measuring perform- 
ance, 126 

nature and measurement of 
sounds, 56-59 
observers, 93 
power measurements, 94 
presentation methods, 82-85, 93 
random order tests, 82 
resonance peak tests, 130-132 
scope of tests, 55 
scoring performance, 94 
signal and background record- 
ings, 56, 93 
signal selector, 80, 93 
sound range recorder, 94, 95 
sounds studied, 93 
sources of error, 56 
techniques, 93-94 
UCDWR tests, 92-128 

Masking of target sounds, labora- 
tory tests, results 
audibility of tones in the pres- 
ence of distributed back- 
ground, 76 

critical band width, 95, 98 
effect of filters, 56, 85-87 
effect of loudness level, 87-88 


energy content of sounds, 56 
estimated audibility threshold, 
59-61 

fluctuating sounds, 95 
frequency translation of airplane 
noise, 132-133 

masking of pure tones, 122-125 
percentage recognition, 61 
prediction of signal-to-noise ra- 
tios at primaudibility, 95 
recognition differentials, 61-78, 
95-104 

signal-to-noise ratios, 76 
simulated hydrophone directivity, 
88-92 

sonic propeller sounds, 104-117 
spectra of signal and background 
noise, 56 

subjective reactions, 125-128 
supersonic propeller sounds, 117- 
122 

transition curves, 78-81 
Masking tone, definition, 17 
Masking-limited listening, 52 
Middle ear, 1, 10-11 

asymmetrical transfer character- 
istics, 11 

nonlinearity of transfer charac- 
teristics, 11, 26, 35 
reflex tensing, 15 
sound distortion, 11 
Mine detection with FM signals, 
169 

Minimal increments in masking 
tests, 126 
Modulation 

see also Auditory motion; Pro- 
peller sounds 
advantage, 154 
background, 154 
effect on signal audibility, 114 
recognition levels of modulated 
sounds, 154 
signal, 153 

Monaural threshold, 14, 15 
Monitoring ship sounds, apparatus, 
93 

Multiple pulses, audibility, 202 


Nerve fibers, 9 

Nerve potentials, electrical, 8-9 
Nerve response, 9 
New London Laboratory 
see CUDWR-NLL tests 
Noise background, 34, 46-48, 156, 
261 


see also Background sound; Pro- 
peller sounds, sonic; Propel- 
ler sounds, supersonic 
airplane noise, 37, 46, 132-133, 153 
ambient noise, 46-48, 187 



rPRIGTED 




cavitation noise, 39, 57, 117, 153 
classifying differences, 47 
deep-sea ambient, 46, 117 
electrical noise, 46, 153 
ground noise of recording me- 
dium, 103 

physiological noise, 15 
reduced by hydrophone directiv- 
ity, 50 

self-noise, 46-47, 103, 201 
sources, 46-48 

Noise intensity measurement, 41 
Noise masking of echoes, 170-208 
Noise masking of echoes, signals 
AM and FM signals, 182-185 
CW pulses, 182, 185, 190-193 
double pulse, 182 
pulse length, 171-182, 203-205 
pulse repetition frequency, 182, 
202-203, 206-208 
pulse type, 203-206 
recorded echoes, 196-198 
rectangular pulses, 185, 188-189 
repeated pulses, 200-208 
rounded pulses, 189-190 
short pulses, 180 
signal envelope, 180 
“signature” of pulses, 180 
single pulses, 188-200 
thermal noise pulses, 205 
Noise masking of echoes, test meth- 
ods 

apparatus, 170 
British tests, 186-188 
BTL tests, 170-185 
CUDWR-USRL tests, 185-186 
familiarity with signal, 208 
sampling effect, 173, 186 
scoring, 171, 176 
self-administered test, 193 
UCDWR tests on repeated pulses, 
200-208 

UCDWR tests on single pulses, 
188-200 

Noise masking of echoes, test re- 
sults 

band widths, 171-182, 188, 192 
compared with reverberation 
masking, 225-226 
distortion of pulse envelope, 175 
echo injection, 186 
environmental sounds, 185 
filters, 171, 173, 175, 206 
heterodyne frequency, 193-196 
intensity levels, 178, 181 
limiters, 199 

loudness loss due to auditory 
build up time, 178 
masking background, 170 
masking efficiency, 187 
masking of noise by noise, 205 


278 


INDEX 


primaudible ratios, 187 
receiver nonlinearity, 198 
recog:nition differentials, 179, 186, 
190-193 

response time of the ear, 179 
smooth and rough background 
noise, 206 

strength of just detectable echo, 
187 

system distortion, 198-200 
transition curves, 171 
Nonlinear gear, 199 
Nonlinearity of auditory mecha- 
nism, 11, 26, 35 
Notch filter, 249 

Oceanographic factors, effect on re- 
verberation decay rate, 166 
Octave band-pass filters, 56, 76-78, 
85, 113 

Off-beam echoes, 162, 164 
Operator fatigue, effect of hetero- 
dyne frequency, 193 
Optimal frequency band, 84, 91, 100 
Ossicles, 1, 2 
Outer ear, 9-10 

amplification of sound, 9 
functions, 9 
resonance effect, 10 
scattering of sound, 15 
Oval window, ear, 2 
Overall level, definition, 58 
Own-doppler, 235, 237 
Own-doppler nullifier (ODN), 236, 
250 

Pain threshold, ear, 17 
definition, 12 
effect of frequency, 13 
Percentage recognition definition, 
61 

Periodmeter, 159, 167 
Phon, 31 

Physiological noise, 15 
Piston type hydrophone, 49 
Pitch discrimination, 2, 28-31 
animal measurements, 29 
detection limits, 30 
effect of localized ear injuries, 29 
frequency limen, 29 
pitch bisection studies, 28 
place theory, 2, 28 
post-mortem studies of deafened 
human ears, 28 

Presentation band, definition, 82 
Presentation differential, definition, 
85 

Priceptible, definition, 52 
Primaudibility 

charts of sonic sounds, 96-97 
definition, 52 


optimal frequency band, 84, 91, 
100 

primaudible pulse loudness in ab- 
sence of masking, 191 
primaudibility ratios, 187 
Primaudible components, 86, 94, 
100, 114 

Propeller sounds, sonic, 104-117, 
149 

aircraft carrier spectrum, 112- 
113 

band width, 110, 114 
band-pass filters, 110 
cavitation, 39 
detection difficulty, 120 
ear discrimination between FM 
and AM, 116 
intensity, 106, 109 
modulation, 106, 114, 116, 153 
propeller thrash, 57 
recorder traces. 111 
“singing” propeller, 40 
shaft noise, 41 
simulation of sound, 104 
spectrum differential, 109 
system tuning. 111 
time pattern, 105 
vibration noise, 40 
Propeller sounds, supersonic, 117- 
122 

detection difficulty, 120 
disadvantages of listening to 
heterodyned sounds, 121 
equipment, 117, 118 
filters, 118 
frequency, 117, 118 
primaudibility, 118 
sound sources, 117 
Pulse loudness, 32-33 
Pulse spectrum, broadening beyond 
critical band, 192 

Pulse spectrum, essential width, 
(definition), 157 
Pulses for echo ranging 
AM pulses, 182-185, 207 
amplitude, 157 

CW pulses, 164, 182, 185, 190-193 
distortion, 159 
double pulses, 182 
duration of pulse, 157 
energy in pulses, 157 
essential width of spectrum, 157 
FM pulses, 159-160, 164, 182-185 
length, 156 
receiver tuning, 159 
rectangular pulses, 188-196, 211- 
213 


repeated pulses, 200-208 
returning echo, 159 
rounded pulses, 213-215 


short pulses, 32-33, 194, 199, 208, 
226 

spectrum, 156-159 
Pure tones, masking 

see Masking of pure tones 
Pure-tone threshold, 60, 122 

Radiation patterns, 42 

see also Hydrophone directivity 
Radiosonic equipment, British tests, 
130 

Random order tests of masking, 82, 
93 

Range tests of reverberation mask- 
ing 

dopplered signals, 247 
echoes without doppler, 223-227, 
231 

Rayleigh probability distribution, 
165, 168 

RCG (reverberation control of 
gain), 232 
RD 

see Recognition differentials 
Receiver gain, effect on signal 
recognition, 87 

Receiver linearity and nonlinearity, 
198 

Recognition differentials 
definition, 61, 153, 260 
effect of background sound, 102 
effect of gain, 124 
effect of test presentation meth- 
ods, 82-85 
800 cycle pulse, 261 
masked target sounds, field tests, 
146-151 

masked target sounds, laboratory 
tests, 61-78, 90-92, 95-104 
masking of tones by tones, 245 
noise masked CW pulses, 190-193 
reverberation masked echoes with 
doppler, 241-246, 262-263 
reverberation masked echoes 
without doppler, 223-233, 246- 
247, 262 

reverberation masked injected 
pulses, 211-215 

reverberation masked recorded 
echoes, 215-217 
Recognition levels 

modulated sounds, 154 
noise background, 261-262 
reverberation background with 
doppler, 262-263 

reverberation background with- 
out doppler, 262 
steady sounds, 154 
transition curves, 155 
Recognition percentage, definition, 
61 


INDEX 


279 


Recognition probability, 61, 79, 129, 
171, 223-233 

Recorded echoes, noise masking, 
196-198 

comparison with rectangular 
pulses, 197 

group performance, 196 
recognition differentials, 197 
sound intensity, 197 
test procedures, 196 
tonality, 197 

Recorded echoes, reverberation 
masking, 215-217 
gate circuit, 215, 216 
round gate and square gate 
echoes, 216 

Recorded sea reverberation, 209, 
251 

Recording listener response, 94 

Rectangular pulses 
effect of heterodyne frequency, 
193-196 

noise masking, 188-193 
recognition differentials with 
short pulses, 194 

recognition levels for 400 cycle 
pulses, 193 

reverberation masking, 211-213 

Remote masking, 18-20, 35, 241-244, 
258 

Repeated pulses, noise masking, 
200-208 

factors affecting, 200, 202 
heterodyne frequency, 200 
irregular pulse repetition, 200 
pulse generation, 200 
pulse repetition frequency, 202- 
203 

pulses studied, 200 
recognition differentials, 261 
recorded masking background 
noise, 201 
signal level, 200 
test procedure, 200, 202 

Resonance, hydrodynamic theory, 
6-7 

Resonance peak tests of masking, 
130-132 

ability to detect peaks, 131 
test procedure, 132 
transition curves, 132 

Resonance theory of hearing, 2-7 

Response time of the ear 
see Ear, response characteristics; 
Ear structure 

Reverberation background, 164-169 
amplitude measurements, 221 
bottom, 166 

CW reverberation, 165-168 
electrical measurement, 229 


FM reverberation, 168-169 
intensity, 165, 225 
level, 211 

similarity to smear echoes, 165 
spectrum obtained with station- 
ary projector, 166 
Reverberation masking of echoes 
with doppler, 234-259 
Reverberation masking of echoes 
with doppler, test methods 
British tests, 225, 237-251 
experimental procedure, 238 
headphones, 238-240, 250-251 
learning effect, 255, 256 
loudspeaker, 250-251 
test administration, 240-241 
test apparatus and procedure, 
251-253 

Reverberation masking of echoes 
with doppler, test results 
AVC, 240 

comparison with masking of one 
tone by another, 242 
conditions for resolution of spec- 
tra, 235 

doppler effect, 234-237 
doppler shifts, 242, 244, 247 
effect of range, 247, 255-256 
effect of signal frequency and 
duration, 253-255 
film recorded sea reverberation, 
251 

gain control, 240 

generation of subjective har- 
monics, 243 

heterodyne frequency, 248-250 
level stabilizer, 240 
limit for auditory determination 
of doppler shift, 243 
masking curves obtained in tests, 
241 

masking of one tone by another, 
242, 245 

own-doppler nullifier, 250 
recognition differentials, 241-245, 
262-263 

recorded echoes, 256 
remote and adjacent masking, 
244, 258 

signal threshold levels, 246 
threshold levels for pulses with 
large doppler shifts, 244 
volume control, 240 
width of reverberation spectrum, 
237 

Reverberation masking of echoes 
without doppler, 209-233 
Reverberation masking of echoes 
without doppler, signals 
echo and reverberation record- 
ings, 215-220 


injected pulses, 211-215 
pulse length, 225 
recorded reverberation, 209 
rectangular pulses, 211-213 
rounded pulses, 213-215 
weak echo, 222 

Reverberation masking of echoes 
without doppler, test methods 
BTL tests, 209-217 
cautious and uncautious observ- 
ers, 214 

commissive errors, 222, 252 
effect of test conditions on re- 
sults obtained, 210 
fatigue in testing, 221 
fixed range tests, 223-227 
initial contact experiments, 231 
injection range unknown to sub- 
ject, 231 

loudspeaker or headphones, 233 
memorization effect, 222 
number of reverberation samples 
used on each test, 221 
personnel, 222 
reliability of tests, 210 
test procedure, 209-211, 217, 221- 
222 

training effects on personnel, 

222- 223 

UCDWR tests, 217-233 
variable range tests, 231 

Reverberation masking of echoes 
without doppler, test results 
compared with noise masking of 
echoes, 225-226 
distortion effects, 232-233 
gate circuit, 215-216 
loudness level, 231-232 
measurement of signal and rever- 
beration level, 211, 220 
prediction of detection probabil- 
ity, 231 
RCG, 232 

recognition differentials, 223-233, 
246-247, 262 

recognition probability curves, 

223- 233 

reverberation intensity measure- 
ment, 225-229 

signal and reverberation level 
measurements, 220-221 
signal threshold levels, 246 
time-amplitude pattern, 220 
variability of individual recogni- 
tion differentials, 227-231 
variations in reverberation pitch, 
209 

Reverberation recordings, 209, 218 
blobs, 219 

change of pitch, 218 
loss of tonality, 219 


280 


INDEX 


Reverberation spectrum spread pro- 
duced by own-doppler, 237 
Reverberation-controlled g’ain 
(RCG), 240 
Round window, ear, 2 
Rounded pulses, noise masking, 189- 
190 

Rounded pulses, reverberation 
masking, 213-215 
band-pass filter, 213 
effect of reflection, 215 
RD probability corrected for ob- 
server cautiousness, 214 
Round-gate echoes, 216 

Secondary lobe, 49 
Selective attenuation of under- 
water sound, 45, 56 
Self-noise of a listening vessel, 46- 
47, 103, 201 
Sensation level, 17, 26 
Sensory and nervous structures of 
the ear, 7-8 
Shadows, acoustic, 42 
Shaft rate modulation, 153 
Ship signals, amplitude modulation, 
98 

Ship sounds, 40, 109 

aircraft carrier spectrum, 112- 
113 

body rumble, 42 
crew activities noise, 42 
detectability, 43 
intensity, 43 
machinery noise, 41-42 
primaudibility, 96-97 
propeller noise, 39 
pumps, compressors, diesel ex- 
hausts, 42 

ranges measured at, 39 
self-noise, 46, 103 
sources, 39 

wave slapping against hull, 42 
Short pulses 

detection, 199, 208 
loudness, 32-33 
observed pitch, 194 
recognition differentials, 194, 226 
security with, 208 
Shrimp noise, 48 

effect on recognition, 102 
spectrum, 93 
Signal 

see also Echoes; Pulses for echo 
ranging; Target sounds 
definition, 17 

measurement of signal level, 220 
modulation, 153 
spectrum, 153 


Signal detection 

see also Noise masking of echoes; 
Reverberation masking of 
echoes with doppler; Rever- 
beration masking of echoes 
without doppler 
critical-band rule, 76 
factors affecting, 37, 43 
probability, 78 
transition curves, 78-81 
Signal envelope, 180 
Signal filter 
see Filters 
Signal masking 

see Noise masking of echoes; Re- 
verberation masking of 
echoes with doppler; Rever- 
beration masking of echoes 
without doppler 

Signal recognition level, 153, 225, 
254 

Signal recognition probability, 155 
Signal threshold levels, 246 
Signal-background mixture, 154, 
261 

Signature of pulses, 180 
Smear echoes, 162, 164, 225 
Sonar bearings, 139 
Sonar operators, 79 
doppler drill, 125 
training, 84, 222 
Sonic listening gear, 46 
JP-1 hydrophone, 135 
Sonic noise audibility, effect of sig- 
nal spectrum, 146-150 
Sonic propeller sounds 

see Propeller sounds, sonic 
Sonic sounds, primaudibility, 96-98 
Sound, energy content, 56 
Sound equipment, types, 37 
Sound power distribution over es- 
sential spectrum, 192 
Sound range recorder, 94 
Sound School, 125 
Sound spectrum, 112-113 
aircraft carrier, 43 
background, 153 
50 cycle spectra, 94 
ideal pulses, 156-159 
rectangular pulses, 211-212 
signal and background, 153 
spectrum level, 56, 58, 77 
submarines, 41, 42 
Sound velocity in sea water, 134 
Sounds, complex 
see Complex sounds 
Spectrum differential 
definition, 115 

effect of background type, 116 
effect of loudness level, 116-117 


for sonic and supersonic propeller 
sounds, 122 

ship speed frequency modulation, 
116 

Spectrum level, 56, 58, 77 
Specularly reflected sound, 162 
Spiral ligament, ear, 3 

tension exerted upon auditory 
strings, 4 

Square-wave modulation of the sig- 
nal, 147 

Standard reference band, definition, 
58, 125 

Steady sounds, recognition levels, 
154 

Stimulation pattern on the basilar 
membrane, 22, 28, 35 
see also Thresholds of the ear 
adjacent masking, 19, 20, 242 
beats, 20-28 
damping, 4 
Structure of the ear 
see Ear structure 

Subjective reactions, masking tests, 
125-128 

effect of experience, 125, 126 
effect of signal interruptions, 127 
effect of signal modulation, 126, 
127 

effect of training, 125 
fatigue, 125, 128 
individual differences, 126 
memory effect, 127 
method of measuring perform- 
ance, 126 

Submarine detection of aircraft, 
acoustic, 132-133 

Submarine signals, amplitude mod- 
ulation, 98 

Submarine sound operator, 47 
Submarine sounds, 40, 109 
gear noise, 42 
primaudibility, 96-97 
recognition differentials, 69 
self-noise, 201 

Subsonic range of underwater 
sound, 133 

Supersonic listening gear, disc hy- 
drophone, 135 

Supersonic propeller sounds 

see Propeller sounds, supersonic 
Supersonic sounds, primaudibility, 
119-120 

Sweep rate, hydrophone, 136-140 

Target bearing, 48 
Target course determination by 
doppler shift, 236 
Target doppler, 236, 242 
Target identification, 40, 48, 155 


INDEX 


281 


Target noise, masking efficiency, 
187 

Target sounds, 37-46 
see also Ship sounds; Submarine 
sounds 

factors affecting, 37 
far from source, 44-46 
interference patterns, 45 
modulation, 153 
selective attenuation, 45 
spectrum, 153 
Target sounds, masking 

see Masking of target sounds, 
field tests; Masking of target 
sounds, laboratory tests 
Target speed 

determined by doppler shift, 236 
determined by propeller beats, 40 
Tectorial membrane, ear, 7 
sound distortion, 11 
Test methods and results 

see Masking of target sounds, 
field tests ; Masking of target 
sounds, laboratory tests; 
Noise masking of echoes; Re- 
verberation masking of 
echoes with doppler; Rever- 
beration masking of echoes 
without doppler 
Thermal noise, 180, 201, 205 
Threshold shifts due to masking, 
17-19 

binaural effect, 19 
effect of beats, 20 
effect of time, 19 
frequency effects, 19 
measuring method, 17 
Thresholds of the ear 

absolute audibility threshold, 12- 
16, 59-61 

binaural threshold, 15 
bone conduction threshold, 16, 19 
free-held threshold, 14-16, 52, 122 
headphone threshold, 14-16, 52, 
60, 238-240 

masked threshold, 17-28, 52, 83 
monaural threshold, 15, 17 


pain threshold, 17 
threshold for distributed sounds, 
59, 60 

tone threshold, 18, 59 
Threshold-limited listening, 52 
Through-the-hull (TTH) gear, 135 
Time-amplitude patterns 
see also Auditory motion 
effect of pulse repetition rate, 
206-208 

hydrophone directivity, 136-139 
of reverberation-masked dop- 
plered echoes, 219 
of reverberation-masked echoes 
without doppler, 220 
of ship signals, 43-46 
of sonic propeller sounds, 105 
of thermal noise, 112, 180 
of water noise, 143-144 
peak factor, 58 

periodmeter analysis of pulses 
and echoes, 159-164 
time constant of measuring in- 
strument, 68 

types of time patterns, 48 
Time-varied gain (TVG),240 
Tone audibility in the presence of 
distributed backgrounds, 76 
Tone discrimination, 28-34 
loudness, 31-34 
pitch, 28-31 
Tone thresholds 

see Thresholds of the ear 
Torpedo noise, 40 
Transition curves 
audibility of sonic noises, 62-75, 86 
definition, 61, 155 
distortions due to guessing, 79 
effect of listening level, 232 
effect of observer experience, 81 
effect of observer’s attitude, 79 
effect of presentation interval, 129 
effect of size of group, 81 
elimination of guessing, 79 
method of test presentation, 82- 
85 

rectangular pulses, 212 


reverberation-masked dopplered 
signals, 252 

reverberation-masked echoes, 223, 
224 

rounded pulses, 213-215 
spread, 80, 81, 155, 176 

UCDWR tests 

masking of target sounds, 92-128 
noise masking of repeated pulses, 
200-208 

noise masking of single pulses, 
188-200 

reverberation masking of echoes 
with doppler, 251-259 
reverberation masking of echoes 
without doppler, 217-233 
Underwater sound gear 
see also Hydrophone 
acoustic baffle, 49, 50 
apparatus for monitoring ship 
sounds, 93 

disc hydrophone, 135 
domes, 46, 135 
echo-ranging gear, 37 
listening gear, 37, 48-52 
JP-1 hydrophone, 131-144 
nonlinear gear, 199 
sonic listening gear, 46, 135 
supersonic listening gear, 135 
TTH gear, 135 

Underwater Sound Reference Lab- 
oratory 

see CUDWR-USRL tests 
Undopplered pulses 

see Reverberation masking of 
echoes without doppler 
University of California Division 
of War Research 
see UCDWR tests 
Up-doppler, 235, 241 

Water reverberation, 166 
Water noise, 60 
‘‘Well-balanced” sound, 60 
“White” noise, 60 

Wideband masking background, 194 


DECL ASSIFIED 
By authority Secretary of 

SEP 7 I960 

Defense memo 2 August 1960 


LIBRARY OF CONGRESS 




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